Ensure behavior-affecting headers are excluded when forwarding proxies/openapi

[jlowin]

Closes #601

[Copilot]

Pull Request Overview

This PR standardizes on using x-server-header in tests, expands the default headers excluded by get_http_headers(), and updates documentation to reflect the new exclusions.

• Tests in test_openapi.py now use x-server-header instead of X-SERVER and add checks around excluded headers.
• The get_http_headers dependency in dependencies.py grows its exclusion set (including proxy-related headers) and enforces lowercase.
• Documentation in http-requests.mdx is updated to mention host and content-length exclusions.

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
tests/client/test_openapi.py	Renamed headers in tests, added a new excluded-header test, and left a large commented block.
src/fastmcp/server/dependencies.py	Expanded exclude_headers set with additional headers and added an assert for lowercase enforcement.
docs/patterns/http-requests.mdx	Updated prose to reflect that host and content-length are excluded by default.

Comments suppressed due to low confidence (2)

tests/client/test_openapi.py:193

• [nitpick] The test name suggests that the header is ignored, but the assertion checks it’s overwritten. Consider renaming to reflect the actual behavior (e.g. test_client_excluded_header_is_overwritten).

async def test_client_with_excluded_header_is_ignored(self, sse_server: str):

src/fastmcp/server/dependencies.py:51

• You’ve added several new headers (connection, proxy-authorization, etc.) to the exclusion set. Consider adding tests to verify that each type of header is actually stripped.

exclude_headers = {