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