fix: propagate parameter descriptions in create_tool_call

[jjovalle99]

Summary

• _get_function_parameters mutates field_info.description after FieldInfo
  construction (line 137). Pydantic v2 ignores this mutation in
  model_json_schema() because it only reads attributes passed at Field()
  construction time. Pydantic documents this as "only accidentally working"
  since _attributes_set is not updated. All docstring-derived parameter
  descriptions are silently dropped from the generated tool schema.
• For fresh FieldInfos (no existing annotation), pass description= directly
  to Field() at construction.
• For existing FieldInfos (from Annotated[T, Field(...)] or = Field(...)
  defaults), override description via Annotated stacking. Pydantic merges
  stacked metadata left-to-right,
  so the description Field appended last takes precedence while preserving
  existing constraints. This is the same pattern Pydantic uses in
  rebuild_annotation().
  No private APIs.

Fixes #471

Test plan

• 17 new tests in src/mistralai/extra/tests/test_tools.py
• Covers: docstring descriptions, Annotated with constraints, Field-as-default,
  shared FieldInfo instances, required vs optional params, no-docstring edge cases
• 10 of 17 fail on current main, 17/17 pass with this fix