Ensure content-length is always stripped from client headers

[jlowin]

Closes #586

[Copilot]

Pull Request Overview

This pull request ensures that the client headers forwarded to the remote server never include the problematic "content-length" header by replacing header extraction methods and updating relevant tests and documentation.

• Updated header extraction in server and client modules to use get_http_headers() instead of get_http_request() based helpers
• Removed obsolete _get_mcp_client_headers() function and updated documentation to reflect these changes

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
tests/client/test_openapi.py	Removed an unnecessary client.ping() call
src/fastmcp/server/openapi.py	Replaced legacy header extraction with get_http_headers()
src/fastmcp/server/dependencies.py	Introduced get_http_headers() with header stripping logic
src/fastmcp/client/transports.py	Updated to use the new header extraction via dict union operator
docs/patterns/http-requests.mdx	Updated documentation to use and explain get_http_headers()