Remove explicit SSE content encoding

[DragonFSKY]

Motivation and Context

Fixes #1574.

SSE responses currently emit Content-Encoding: identity even though no content coding is applied. RFC 9110 section 8.4 defines Content-Encoding as the list of content codings that were applied to the representation and notes that identity is reserved for Accept-Encoding, so it SHOULD NOT be included in Content-Encoding.

Since no SSE content coding is applied here, the response can omit the header while still preserving the existing SSE buffering controls.

Changes

• Removes the explicit Content-Encoding: identity assignment from SSE response initialization.
• Updates legacy SSE and Streamable HTTP integration tests to assert that SSE responses do not include a content encoding.
• Keeps the existing Cache-Control, X-Accel-Buffering, and DisableBuffering() behavior.

How Has This Been Tested?

• dotnet test tests/ModelContextProtocol.AspNetCore.Tests/ModelContextProtocol.AspNetCore.Tests.csproj -f net10.0 --filter "FullyQualifiedName~EventSourceResponse_Includes_ExpectedHeaders"
  • RED before the production change: 3 failed because the collection contained identity.
  • GREEN after the production change: 3 passed.
• dotnet test tests/ModelContextProtocol.AspNetCore.Tests/ModelContextProtocol.AspNetCore.Tests.csproj -f net10.0 --no-build --no-restore
  • 377 passed, 18 skipped.

Breaking Changes

None.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository style guidelines
• New and existing relevant tests pass locally
• I have added appropriate regression coverage
• Documentation updates were not needed for this header-only behavior fix