fix(fetch): make fetch tool schema Gemini/OpenAPI 3.0 compatible

[Dharit13]

Summary

Fixes the fetch server's tool schema to be compatible with LLM providers that only support OpenAPI 3.0 schema keywords (e.g. Google Gemini 2.5 Pro), which currently reject the schema with a 400 INVALID_ARGUMENT error.

Two incompatibilities are addressed:

1. max_length field used gt=0 / lt=1000000 Pydantic constraints, which generate exclusiveMinimum / exclusiveMaximum in the JSON Schema. Gemini does not recognize these keywords. Changed to ge=1 / le=999999 — identical semantics for integers — which emits the universally supported minimum / maximum keywords instead.

2. url field used AnyUrl, which generates format: \"uri\" and minLength: 1. Gemini only supports \"enum\" and \"date-time\" for string format, and does not support minLength. Added a WithJsonSchema override to emit a plain {"type": "string"} schema while preserving AnyUrl runtime validation — so invalid URLs are still rejected with a proper error, but the schema no longer contains unsupported keywords.

Fixes #1624

Changes

src/fetch/src/mcp_server_fetch/server.py

• max_length field: gt=0 → ge=1, lt=1000000 → le=999999
• url field: added WithJsonSchema({"type": "string", "description": "URL to fetch"}) to override schema output while keeping AnyUrl runtime validation

src/fetch/tests/test_server.py

• Added TestFetchToolSchema.test_schema_uses_inclusive_bounds — asserts minimum/maximum present, exclusiveMinimum/exclusiveMaximum absent
• Added TestFetchToolSchema.test_url_schema_omits_unsupported_keywords — asserts format and minLength absent from url schema

Evidence

Live Gemini validation was run against gemini-2.5-pro using the exact generated tool schema sent to generateContent.

Pre-fix schema (reproduced locally):

• HTTP 400
• INVALID_ARGUMENT
• Error excerpt:
  • Unknown name "exclusiveMaximum"
  • Unknown name "exclusiveMinimum"

Current PR schema:

• HTTP 200
• Gemini accepted the tool declaration payload
• Response text: Acknowledged.

Test plan

• All 22 tests pass (uv run python -m pytest tests/test_server.py -v)
• Schema verified: no exclusiveMinimum, exclusiveMaximum, format, or minLength in output
• AnyUrl runtime validation confirmed: valid URLs accepted, invalid URLs rejected with ValidationError
• Live Gemini validation: sent the exact generated schema to gemini-2.5-pro via generateContent; request was accepted with HTTP 200 and returned Acknowledged.
• No behavioral change — same validation semantics, only schema representation changed