fix: wire up dereference_refs() in tool schema pipeline

[jlowin]

PR #2861 backported dereference_refs() to v2 but never actually called it — the function exists and is tested in isolation, but nothing in the tool schema pipeline invokes it. As a result, $ref references are sent as-is to MCP clients, breaking clients like VS Code Copilot that don't handle them.

This wires it up by adding a dereference parameter to compress_schema() and enabling it at the two tool schema creation sites (input and output schemas in Tool.from_function). Schemas are now inlined before being sent to clients.

# compress_schema now accepts dereference=True to inline $ref
schema = compress_schema(schema, prune_titles=True, dereference=True)

Closes #3153

[coderabbitai]

Walkthrough

The change adds an optional dereference step to JSON Schema compression and enables it when generating both input and output schemas in ParsedFunction.from_function. compress_schema gained a dereference: bool = False parameter and, when true, inlines $ref by calling dereference_refs(). A helper _strip_discriminator_mappings was added and is invoked after dereferencing to remove discriminator.mapping entries that reference removed $defs. ParsedFunction.from_function now calls compress_schema(..., dereference=True) for input and output schemas.

Possibly related PRs

• Backport: Dereference $ref in tool schemas for MCP client compatibility #2861: Adds the dereference_refs() implementation that compress_schema(..., dereference=True) invokes.
• Dereference $ref in tool schemas for MCP client compatibility #2808: Modifies compress_schema usage in tool.py to enable $ref dereferencing during schema generation.
• Dereference $ref in tool schemas for MCP client compatibility #2814: Integrates JSON Schema dereferencing into compress_schema and the tool/schema generation flow, aligning with the current inlining approach.

🚥 Pre-merge checks | ✅ 6

✅ Passed checks (6 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately describes the main change: wiring up the existing dereference_refs() function into the tool schema pipeline.
Description check	✅ Passed	The description clearly explains the issue, solution, and provides code examples, though contributors checklist is not explicitly checked.
Linked Issues check	✅ Passed	The PR directly addresses issue #3153 by implementing the missing call to dereference_refs() in the schema pipeline for both input and output schemas.
Out of Scope Changes check	✅ Passed	All changes are scoped to enabling dereferencing in the schema pipeline: adding dereference parameter to compress_schema() and wiring it into Tool.from_function().
Docstring Coverage	✅ Passed	Docstring coverage is 80.00% which is sufficient. The required threshold is 80.00%.
Merge Conflict Detection	✅ Passed	✅ No merge conflicts detected when merging into release/2.x

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✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch fix/v2-wire-dereference-refs

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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve discriminator mappings when inlining $ref schemas

Calling compress_schema(..., dereference=True) now runs dereference_refs, which removes $defs after resolving only $ref nodes; this breaks discriminated-union schemas because Pydantic emits discriminator.mapping values like #/$defs/Cat as plain strings, so those mappings become dangling once $defs is stripped. For tools whose input/output annotations use discriminated unions, clients/validators that rely on discriminator.mapping can no longer resolve variants correctly.

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