Google provider: JSON Schema 'const' in tool schemas causes Pydantic/Gemini errors

[xek]

Running with the Google provider fails because tool parameter schemas include JSON Schema 'const'. This triggers either Pydantic validation errors or a Gemini API 400.

To reproduce:

1. Run:
   uvx fast-agent-mcp go --model google.gemini-2.5-pro --uvx wcgw
2. Type: hi

Observed errors

• Pydantic validation (before provider request):
  ValidationError: ... properties.*.const ... Extra inputs are not permitted

• Gemini API (after getting past Pydantic):
  Google API Error: 400
  Invalid JSON payload received. Unknown name "const" at ...parameters...

Environment

• OS: Linux (Fedora) x86_64
• Python: 3.13
• uv: 0.8.23
• fast-agent-mcp: latest via uvx
• Model: google.gemini-2.5-pro

Notes

• Offending places include fields like allowed_globs / allowed_commands that use const: "all".
• google.genai function_declarations don’t accept 'const'; Pydantic may also reject it.

Workaround verified locally

• In src/fast_agent/llm/provider/google/google_converter.py:
  • If const is a string, rewrite to enum: [value].
  • If const is non-string, drop the constraint to avoid Gemini 400.
• With this, the repro initializes and responds.

Proposal

• Update _clean_schema_for_google to rewrite/remove 'const' as above (and continue stripping other unsupported keys).