Parse multi-line Modern SSE events

[kocaemre]

Summary

• vMCP's Modern backend client parsed each SSE data: line as a standalone JSON-RPC envelope, but the SSE spec defines one event as all data: lines up to the blank-line boundary joined with \n.
• This meant a conformant Modern backend that split one JSON response across multiple data: lines could be misread as not-Modern and fall back to Legacy.
• Update readModernSSE to accumulate one SSE event at a time, join multi-line data: fields, ignore interleaved server-to-client messages, and flush the final event even without a trailing blank line.
• Extend the Modern SSE response tests to cover single-line events, multi-line data:, CRLF line endings, interleaved notifications, and EOF without a trailing blank line.

Fixes #6102

Type of change

• Bug fix
• New feature
• Refactoring (no behavior change)
• Dependency update
• Documentation
• Other (describe):

Test plan

• Unit tests (task test)
• E2E tests (task test-e2e)
• Linting (task lint-fix)
• Manual testing (describe below)

Manual testing:

• RED before fix: PATH=/usr/local/go/bin:/root/go/bin:$PATH go test ./pkg/vmcp/client -run TestModernCall_SSEResponse failed on multi-line data response with backend response is not a Modern (2026-07-28) MCP response.
• GREEN after fix: PATH=/usr/local/go/bin:/root/go/bin:$PATH go test ./pkg/vmcp/client -run TestModernCall_SSEResponse
• PATH=/usr/local/go/bin:/root/go/bin:$PATH go test ./pkg/vmcp/client -run 'TestModernCall_SSEResponse|TestModernCall_ErrorMapping|TestModernCall_Decode'
• PATH=/usr/local/go/bin:/root/go/bin:$PATH go test -race ./pkg/vmcp/client -run TestModernCall_SSEResponse
• PATH=/usr/local/go/bin:/root/go/bin:$PATH go test ./pkg/vmcp/client
• PATH=/usr/local/go/bin:/root/go/bin:$PATH task lint
• git diff --check

I also ran PATH=/usr/local/go/bin:/root/go/bin:$PATH task test. It failed in unrelated packages (cmd/thv/app, pkg/client, and pkg/skills/skillsvc) outside this PR's two touched files, including a panic in pkg/client/config_test.go; the targeted pkg/vmcp/client package and lint checks above passed.

API Compatibility

• This PR does not break the v1beta1 API, OR the api-break-allowed label is applied and the migration guidance is described above.

Does this introduce a user-facing change?

No direct user-facing CLI/API change. It fixes vMCP's backend revision detection for conformant Modern streamable-HTTP servers that split SSE data: fields.

Special notes for reviewers

The parser keeps the existing behavior of only returning the response matching the JSON-RPC id requested by modernCall; server-to-client requests/notifications and other response ids are still ignored.