fix(tools): dereference draft-07 definitions in MCP tool schemas

[gaoflow]

Summary

McpToolset crashes with KeyError: 'definitions' when an MCP server returns a tool inputSchema written in JSON Schema draft-07 — i.e. using definitions and $ref: "#/definitions/..." instead of the draft 2019-09+/2020-12 $defs. The MCP specification (2025-11-25) explicitly permits draft-07 schemas, so ADK should accept them.

Fixes #5940 (the same crash was previously reported in #3685 and closed as stale).

Root cause

_dereference_schema in src/google/adk/tools/_gemini_schema_util.py only looked up $refs in the $defs block and only stripped $defs after resolving:

defs = schema.get("$defs", {})
...
if "$defs" in dereferenced_schema:
    del dereferenced_schema["$defs"]

For a draft-07 schema there is no $defs, so defs is empty, every $ref: "#/definitions/..." is left unresolved, and the definitions block survives untouched. _to_gemini_schema then passes that leftover definitions keyword into Schema.from_json_schema(...), which raises KeyError: 'definitions'.

Fix

Resolve refs against both definitions (draft-07) and $defs (draft 2019-09+), and strip both blocks once resolved. $defs wins on the (pathological) key collision. Draft 2019+ schemas are unaffected because definitions is simply absent.

defs = {**schema.get("definitions", {}), **schema.get("$defs", {})}
...
for defs_keyword in ("$defs", "definitions"):
    if defs_keyword in dereferenced_schema:
        del dereferenced_schema[defs_keyword]

The $ref lookup itself already keys on the final path segment (ref_uri.split("/")[-1]), so #/definitions/Pet and #/$defs/Pet both resolve to Pet without further changes.

Testing

Added test_to_gemini_schema_draft_07_definitions_and_ref, mirroring the existing $defs test but using draft-07 definitions + $ref: "#/definitions/...". It fails on main with KeyError: 'definitions' and passes with this change. The full tests/unittests/tools/test_gemini_schema_util.py suite (now 69 tests) passes, confirming no regression for the $defs / circular-ref / reused-ref cases.

Thanks to @rrazvd for the reproduction and definitions → $defs workaround in the issue thread.

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