mcp: better handle 405 on GET requests

[nacx]

Description

Changes how the GET requests are managed when some backends return 405.

The main challenge is that the GET implementation always returns a 202, no matter what the upstream backends return. When all upstream backends return a 405, that translates into an immediate 202 completed, and clients reconnect, causing the constant reconnection loop.

Ideally, when all backends return 405, we should return a 405 as well, but this is challenging: it would require for us to not return the response status until we have received a 405 from all backends, and if some backend does not return and it's just waiting to start streaming events, we may not be able to return the right response code anything until we've received the first event, which may be waiting too long.

This PR addresses it in a different way, by always returning a 202 and keeping the stream open between the client and the gateway, even when all backends return 405, or they return EOF after stopping sending events. This prevents the client reconnection loop and keeps the stream open to send gateway-related notifications, such as tool changes. Even though upstream backends don't have more events to report, the gateway itself may still use it to send notifications, so I think it is OK to keep it open and move the control to the client, and let clients decide when to close the stream or reopen it.

Related Issues/PRs (if applicable)

Fixes #1924
Fixes #1925

Special notes for reviewers (if applicable)

N/A

[nacx]

@mathetake WDYT about this approach?

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.37%. Comparing base (2d35d43) to head (ee65bd4).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1980      +/-   ##
==========================================
+ Coverage   84.33%   84.37%   +0.03%     
==========================================
  Files         130      130              
  Lines       17987    17988       +1     
==========================================
+ Hits        15170    15177       +7     
+ Misses       1873     1868       -5     
+ Partials      944      943       -1     

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[nacx]

/retest