test: add unit tests for the GCF data format (follow-up to #2235)

[blackwell-systems]

Summary

Follow-up to #2235, which added --experimentalDataFormat=gcf. During that review the request was to add unit tests as the prerequisite for GCF being considered for the default/non-experimental path:

"Please also consider adding some unit tests - they are not strictly required for experimental feature, but we'll need them in case we do decide to make GCF the default. This might be a separate PR I think."

This PR adds that coverage. It introduces one new test file, tests/McpResponseGcf.test.ts (8 tests), with no changes to src/.

Scope

Covers every McpResponse-level surface that compactEncode touches:

Encode surfaces — each round-trips losslessly back to structuredContent:

• Page snapshot
• Network requests
• Console messages (including the bracket-colon shapes, e.g. [Error]: net::ERR_CONNECTION_REFUSED)
• Heap snapshot class diffs (array of flat objects)
• Detailed heap class diff (object with nested numeric arrays)

Safety / robustness:

• default format emits no GCF header and still renders the built-in formatter output (guards against a vacuous pass)
• structuredContent is byte-identical with and without GCF — clients consume structuredContent, so enabling GCF must only change the text representation
• Values containing delimiters and GCF sentinels (| , " ~ - @ ##, newlines, tabs, unicode, empty strings, typed-literal lookalikes like true) round-trip losslessly

Rigor

• Tests decode the emitted block with @blackwell-systems/gcf and deep-equal it against structuredContent, so they assert real losslessness rather than snapshotting opaque bytes — a regression in the encode path fails them.
• One test additionally pins the exact wire bytes (golden assertion), so a symmetric encode+decode bug can't round-trip its way past it.
• All 8 pass; verified stable across repeated runs. Lint, type-check, and build are clean. The rest of the suite is unaffected (the only failures locally are the pre-existing e2e tests that require a system Chrome stable channel).

Notes

• No src/ changes; no new runtime dependencies (@blackwell-systems/gcf is already the optional peer dep added in feat: add experimentalGcfFormat flag for GCF-encoded tool responses #2235).
• Tests use the existing withMcpContext harness and mock helpers, matching the conventions in tests/McpResponse.test.ts.

cc @zyzyzyryxy — follow-up to the unit-test request in #2235.