fix(server): connectors header forwading

[pilartomas]

No description provided.

[gemini-code-assist]

Summary of Changes

Hello @pilartomas, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request primarily focuses on improving the robustness and clarity of server-side connector interactions. It enhances the forwarding of essential HTTP headers to connectors, ensuring better communication and context. Additionally, it refines how response headers are handled by selectively returning key session identifiers and updates a connection error message for better user understanding.

Highlights

• Enhanced Header Forwarding: The mcp_proxy method now forwards additional critical headers, specifically mcp-protocol-version, mcp-session-id, and last-event-id, from incoming requests to connectors. This ensures that connectors receive more comprehensive context for processing requests.
• Refined Response Header Handling: The mcp_proxy method has been updated to selectively include only the mcp-session-id in the response headers, if present, rather than forwarding all original response headers. This streamlines the response and focuses on essential session information.
• Improved Error Messaging: An error message for httpx.RequestError during connector connection attempts has been updated to be more descriptive, changing from 'Connector must be in connected or auth_required state' to 'Unable to establish connection with the connector'.

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[gemini-code-assist]

Code Review

This pull request aims to fix header forwarding for connectors. The changes include an improved error message, forwarding additional headers to connectors, and adjusting how response headers are handled. While the changes to the error message and the list of forwarded headers are good, the modification to response header forwarding is too restrictive and could cause issues by dropping important HTTP headers like Content-Encoding. I've provided a high-severity comment with a suggestion to fix this potential bug. I've also added a medium-severity comment to improve the maintainability of the forwarded headers list.

[gemini-code-assist]

This list of headers is getting long. For better readability and maintainability, consider defining it as a module-level constant (e.g., a frozenset for efficient lookups).

For example:

# At the top of the file
_FORWARDED_MCP_HEADERS = frozenset((
    "accept",
    "content-type",
    "mcp-protocol-version",
    "mcp-session-id",
    "last-event-id",
))

# In the method:
# ...
for key in _FORWARDED_MCP_HEADERS:
# ...