Feature add support for fetching OpenAPI schema from a remote FastAPI server

[ww-jermaine]

Feature: Add support for fetching OpenAPI schema from a remote FastAPI server

Describe your changes

• Added a fetch_openapi_from_remote flag to FastApiMCP.
• When enabled and a custom http_client is provided, the MCP server will fetch the OpenAPI schema from /openapi.json on the remote FastAPI server instead of generating it locally.
• Falls back to local OpenAPI generation if the remote fetch fails.
• Added a test (test_remote_openapi.py) to verify this feature.

Issue ticket number and link (if applicable)

Closes #136

Screenshots of the feature / bugfix

N/A (feature is backend, but see test for usage example)

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Checklist before requesting a review

• Added relevant tests (test_remote_openapi.py)
• Run ruff (no lint errors)
• Run mypy (type errors are only due to missing stubs for external/test dependencies, not this feature)
• All tests pass (feature test passes, global coverage is a separate project-wide issue)