Fix Linux prebuilt installs for branch-based llama.cpp releases (#5493)

* allow validation of custom releases to pass

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

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

* require exact source provenance for branch direct linux releases

Mirror validated_checksums_for_bundle so incomplete checksum metadata
on a branch/pull/commit release fails closed with a clear error instead
of silently degrading to the legacy master-as-tag source hydration path
that this PR is meant to eliminate. Guard fires when source_commit,
the exact source archive hash, or a derivable source repo URL is
missing from the approved metadata.

Also set plan.llama_tag to approved_checksums.upstream_tag so the
ensure_converter_scripts fallback and the install fingerprint target
the concrete upstream tag (e.g. b9174) rather than the moving branch
label inferred from asset names (master). Legacy b#### releases are
unaffected: synthetic checksums already set upstream_tag to
bundle.upstream_tag, so the swap is a no-op on that path.

Add parametrized negative regression coverage for the three ways
exact provenance can be incomplete (missing source_commit, missing
exact source archive entry, missing source_repo) and update the
existing branch happy-path test to expect b9174.

* revert llama_tag swap to preserve install identity

Keep plan.llama_tag as bundle.upstream_tag (the branch label inferred
from asset names, e.g. master) rather than overriding it with the
approved metadata upstream tag (b9174). The override broke install
identity in two ways:

1. expected_install_fingerprint hashes upstream_tag = llama_tag, so
   the same release would produce a different fingerprint depending on
   which version of this code resolved it, causing spurious reinstalls
   when users upgrade or roll back.
2. UNSLOTH_PREBUILT_INFO.json reports the value as the user-visible
   record of which release was installed; tools and logs should see
   the requested branch label, not the compatibility tag.

The approved metadata still records upstream_tag = b9174 internally
for source archive lookup; the new assertion makes that explicit.

---------

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

studio/install_llama_prebuilt.py

@@ -1273,16 +1273,36 @@ def direct_linux_release_plan(
         attempts.append(cpu_choice)
     if not attempts:
         raise PrebuiltFallback("no compatible Linux prebuilt asset was found")
+    approved_checksums = synthetic_checksums_for_release(
+        repo,
+        bundle.release_tag,
+        bundle.upstream_tag,
+    )
+    resolved_upstream_tag = bundle.upstream_tag
+    if DEFAULT_PUBLISHED_SHA256_ASSET in bundle.assets and not is_release_tag_like(
+        bundle.upstream_tag
+    ):
+        approved_checksums = load_approved_release_checksums(repo, bundle.release_tag)
+        # Require exact source provenance for branch/pull/commit releases.
+        # Mirrors validated_checksums_for_bundle so incomplete metadata
+        # fails closed instead of degrading to the legacy branch-as-tag
+        # source hydration path that this PR is meant to eliminate.
+        if (
+            not approved_checksums.source_commit
+            or exact_source_archive_hash(approved_checksums) is None
+            or source_clone_url_from_checksums(approved_checksums) is None
+        ):
+            raise PrebuiltFallback(
+                f"approved checksum asset {DEFAULT_PUBLISHED_SHA256_ASSET} for "
+                f"{repo}@{bundle.release_tag} did not contain exact source provenance"
+            )
+        attempts = apply_approved_hashes(attempts, approved_checksums)
     return InstallReleasePlan(
         requested_tag = requested_tag,
-        llama_tag = bundle.upstream_tag,
+        llama_tag = resolved_upstream_tag,
         release_tag = bundle.release_tag,
         attempts = attempts,
-        approved_checksums = synthetic_checksums_for_release(
-            repo,
-            bundle.release_tag,
-            bundle.upstream_tag,
-        ),
+        approved_checksums = approved_checksums,
     )