Decompose CodeMode into composable discovery tools (#3354)

* Decompose CodeMode into composable discovery tools

CodeMode now owns only the execute sandbox. Discovery tools (search,
get_schema, etc.) are composable via the discovery_tools parameter.
Each is a Callable[[GetToolCatalog], Tool] factory.

Ships SearchTool (lightweight name+description results) and SchemaTool
(brief markdown or full JSON schemas by tool name) as built-in defaults.

* Rename to Search/GetSchemas/Tags, add tag filtering, fix bugs

- Rename SearchTool→Search, SchemaTool→GetSchemas, Categories→Tags
- Add tags parameter to Search for LLM-side tag filtering
- Add Tags discovery tool for browsing tools by tag
- Fix shared singleton default factories (now per-instance)
- Fix get_schema full mode returning invalid JSON on partial matches
- Fix "untagged" filter inconsistency between Tags and Search
- Split serialization tests to comply with loq line limit
- Rewrite docs for conceptual clarity

* Add three-tier detail levels, remove default_arguments, rename Tags→GetTags, rewrite docs

* Clean up __all__ exports, return valid JSON for empty full-detail results

docs/docs.json

@@ -163,10 +163,10 @@
                       "servers/transforms/namespace",
                       "servers/transforms/tool-transformation",
                       "servers/visibility",
+                      "servers/transforms/code-mode",
                       "servers/transforms/tool-search",
                       "servers/transforms/resources-as-tools",
-                      "servers/transforms/prompts-as-tools",
-                      "servers/transforms/code-mode"
+                      "servers/transforms/prompts-as-tools"
                     ]
                   },
                   {