Use conformant Accept in dual-era e2e

[kocaemre]

Summary

• The dual-era e2e specs were still proving vMCP bridge behavior without the streamable-HTTP Accept: application/json, text/event-stream header that both MCP revisions require.
• Add WithStreamableAccept() to the vMCP dual-era bridge and concurrent-mixing specs so those proxy paths now run under conformant request headers.
• Add a vMCP bridge assertion that Legacy and Modern response content is identical with and without the SSE-capable Accept header, and update RawMCPClient comments/tests now that SSE response parsing is available.

Fixes #6104

Type of change

• Bug fix
• New feature
• Refactoring (no behavior change)
• Dependency update
• Documentation
• Other (describe): E2E coverage hardening

Test plan

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

Manual testing:

• PATH=/usr/local/go/bin:/root/go/bin:$PATH go test ./test/e2e -run 'TestRawClientBuilders/no_Accept|TestRawClientSSEResponse'
• PATH=/usr/local/go/bin:/root/go/bin:$PATH go test -race ./test/e2e -run 'TestRawClientBuilders/no_Accept|TestRawClientSSEResponse'
• PATH=/usr/local/go/bin:/root/go/bin:$PATH task build
• PATH=/usr/local/go/bin:/root/go/bin:$PATH THV_BINARY=$PWD/bin/thv TOOLHIVE_DEV=true TEST_TIMEOUT=15m LABEL_FILTER='vmcp && dual-era' ./test/e2e/run_tests.sh — 7 passed, 0 failed, 500 skipped
• PATH=/usr/local/go/bin:/root/go/bin:$PATH task lint
• git diff --check

Note: I also tried go test ./test/e2e before exporting THV_BINARY; it failed because the full e2e suite could not find thv on $PATH. The labeled e2e run above used the freshly built bin/thv and 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.

Changes

File	Change
test/e2e/vmcp_dual_era_test.go	Run the vMCP dual-era bridge specs with conformant Accept and add plain-vs-SSE content parity coverage.
test/e2e/dual_era_mixing_test.go	Run concurrent mixed-era proxy traffic with conformant Accept.
test/e2e/mcp_raw_client.go	Update RawMCPClient comments for proxy-bound conformant Accept usage.
test/e2e/mcp_raw_client_test.go	Update the no-default-Accept assertion message.

Does this introduce a user-facing change?

No.

Special notes for reviewers

The production behavior is unchanged. This only flips test traffic to the spec-required Accept header now that RawMCPClient can parse SSE-framed POST responses.