Studio: shareable per-checkpoint preview links (#6486)

* checkpoint preview endpoint

* harden new preview endpoints

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* address review

* Studio preview: pin adapter, guard streaming submit, robust copy-link

Harden the public per-checkpoint preview surface:

- Pin use_adapter=True in the preview payload sanitizer. Otherwise an
  unauthenticated /p caller can POST use_adapter=false, which calls
  disable_adapter_layers() on the shared in-memory model without restoring
  it; since load_model skips reloads for the same checkpoint, every later
  visitor (the page never sends the field) keeps getting base-model output
  instead of the fine-tuned checkpoint. Forcing it on also re-enables a
  previously disabled adapter and no-ops on merged checkpoints.
- Ignore preview-page submits while a response is streaming. The send
  button was disabled but the Enter handler still called requestSubmit(),
  so a second request could start before the first reply landed in msgs and
  reorder the chat history. Both the keydown and submit handlers now honor
  the disabled button.
- Keep the cloudflare-URL polling loop alive across transient startup fetch
  errors instead of letting one rejection halt it.
- Build the copy-link from a backend preview_ref (output dir relative to
  outputs_root, gated on previewability and the two-segment /p route limit)
  so a nested output dir no longer copies a basename-only link that 404s.
  Expose preview_ref on training run summaries.

Add route-level security tests (path traversal, payload sanitization,
asset containment, CSP header, HTML title escaping, streaming lock held
until drained) and preview_ref unit tests.

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* Studio preview: Safari-safe submit and adapter pin only for LoRA

Follow-ups from cross-browser and route simulations:

- Preview page: send the message from a shared send() helper called by both
  the form submit and the Enter key, instead of form.requestSubmit(). The
  latter throws on Safari < 16 and older iOS, which broke Enter-to-send there.
  Verified across Chromium, Firefox and WebKit with Playwright.
- Only pin use_adapter=True when the resolved checkpoint is a LoRA adapter
  (adapter_config.json present); for a merged checkpoint strip it to None.
  A merged model has no adapter to toggle, so forcing it on only produced a
  per-request "not a PeftModel" warning. The cross-request base-model
  contamination fix still holds for LoRA previews.

Add a merged-checkpoint test asserting use_adapter is stripped to None.

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* Studio preview: trim verbose comments

Tighten comments across the preview routes, page, checkpoint helpers, and tests
to short single-line notes; drop ones that just restate the code. No behavior
change (verified comment/docstring-only with comment_tools.py check).

* Harden preview routes for PR #6486

- Return a generic 400 detail on a rejected preview path so the public /p
  route never echoes the absolute install path (the real reason is logged
  server-side instead).
- Strip confirm_tool_calls, session_id and rag_scope in the preview payload
  sanitizer so the public surface stays inert regardless of the tool gate.
- Use Path.is_relative_to for the asset containment check, matching the rest
  of the codebase.
- Add img-src 'self' and font-src 'self' to the preview page CSP.
- Preview page: on a mid-stream error keep the streamed text, flag the break,
  and restore the prompt so the user can retry; drop the unused --font-sans var.

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Lee Jackson <130007945+Imagineer99@users.noreply.github.com>
Co-authored-by: Daniel Han <danielhanchen@gmail.com>

studio/backend/utils/api_errors.py

@@ -125,6 +125,12 @@ def is_anthropic_path(path: str) -> bool:
     return path.startswith("/v1/messages")
 
 
+def wants_api_error_envelope(path: str) -> bool:
+    """True for the OpenAI/Anthropic-compatible surfaces: the ``/v1/*`` mount and
+    the preview ``/p/<run>[/<ckpt>]/v1/*`` mount."""
+    return path.startswith("/v1/") or (path.startswith("/p/") and "/v1/" in path)
+
+
 def error_body_for_path(
     path,
     message,
@@ -183,15 +189,16 @@ def _summarize_validation_errors(errors) -> tuple:
 def install_api_error_handlers(app) -> None:
     """Register validation + HTTPException handlers that emit ``/v1/*`` envelopes.
 
-    Both handlers are global but only transform responses for paths starting with
-    ``/v1/``. Non-``/v1/`` paths reproduce FastAPI's default ``{"detail": ...}``
-    behavior exactly so the Studio frontend keeps working.
+    Both handlers are global but only transform responses for OpenAI/Anthropic-
+    compatible surfaces (see :func:`wants_api_error_envelope`: the ``/v1/*`` mount
+    and the preview ``/p/.../v1/*`` mount). Every other path reproduces FastAPI's
+    default ``{"detail": ...}`` behavior exactly so the Studio frontend keeps working.
     """
 
     @app.exception_handler(RequestValidationError)
     async def _handle_validation_error(request, exc):
         path = request.url.path
-        if path.startswith("/v1/"):
+        if wants_api_error_envelope(path):
             summary, param = _summarize_validation_errors(exc.errors())
             return JSONResponse(
                 status_code = 400,
@@ -211,7 +218,7 @@ def install_api_error_handlers(app) -> None:
         # default http_exception_handler, which returns a bodiless Response.
         if not is_body_allowed_for_status_code(exc.status_code):
             return Response(status_code = exc.status_code, headers = headers)
-        if path.startswith("/v1/"):
+        if wants_api_error_envelope(path):
             detail = exc.detail
             # Already a fully-formed envelope: pass through untouched.
             if isinstance(detail, dict) and ("error" in detail or detail.get("type") == "error"):