赵月辉 / fastgpt-migrated

* fix(mcp): only fall back to legacy SSE transport on a 4xx Streamable HTTP error

The MCP client caught any Streamable HTTP connection error and immediately
retried with the legacy HTTP+SSE transport. For a Streamable-HTTP-only server,
a failure unrelated to protocol support (network error, 5xx, or an error after
a successful initialize) was masked by a misleading SSE error such as
"Non-200 status code (405)", hiding the real root cause.

Follow the MCP spec's backwards-compatibility flow: fall back to SSE only when
the server rejects the Streamable HTTP request with a 4xx (e.g. 404/405),
which signals it does not speak Streamable HTTP. Other errors now surface
as-is, and when the SSE fallback also fails both errors are reported.

* test(mcp): cover conditional SSE fallback behavior

Update the fallback test to use a 4xx StreamableHTTPError and add cases for:
non-HTTP errors and 5xx not triggering fallback, and both errors being
surfaced when the SSE fallback also fails.

* perf: code

* perf: code

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Co-authored-by: archer <545436317@qq.com>