deps: update sse-starlette requirement

Updates the requirements on [sse-starlette](https://github.com/sysid/sse-starlette) to permit the latest version.
- [Release notes](https://github.com/sysid/sse-starlette/releases)
- [Commits](sysid/sse-starlette@v1.6.1...v3.0.4)

---
updated-dependencies:
- dependency-name: sse-starlette
  dependency-version: 3.0.4
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

requirements.txt

@@ -43,7 +43,7 @@ uvicorn>=0.30.0,<1.0.0
 
 # MCP-over-HTTP Support (SSE and HTTP transports)
 fastapi-mcp>=0.4.0,<1.0.0
-sse-starlette>=1.6.1,<2.0.0
+sse-starlette>=1.6.1,<4.0.0
 
 # Security (local dev + CI)
 bandit>=1.7.5,<2.0.0