Fixes #60270.

## Summary

- Accept MCP HTTP SSE `data:` fields when the optional space after the
colon is omitted.
- Add a regression test covering `data:{...}` streamable HTTP responses.

## Tests

- `cargo fmt --check`
- `cargo test -p context_server
test_sse_data_field_without_space_after_colon -- --nocapture`
- `cargo test -p context_server -- --nocapture`

Release Notes:

- Fixed MCP HTTP context servers timing out when SSE data fields omit
the optional space after the colon.

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Co-authored-by: dino <dinojoaocosta@gmail.com>