[Tyrie Vella (tyrielv)]

Problem and Context

Unit tests currently start Git, shell commands, and the mount executable, and use real HTTP and named-pipe connections.
These dependencies require machine setup and place integration failures in the unit-test gate.
The real behavior checks remain important and must not disappear behind mocks.

Merge order: #2162 first, then this PR.
This branch contains the current head of #2162 and adds the test migration on top.
Functional tests use the same Git installation resolver as GVFS.

Changes

• Move 92 existing integration cases into local component fixtures within the existing functional test project.
• Preserve real Git authentication matching, configuration precedence, credential-file locks, worktree settings, and Git/libgit2 comparisons.
• Preserve process timeout, output-callback, argument round-trip, and mount executable exit-code checks.
• Preserve loopback HTTP timeout, status, credential-policy, connection-permit, and cancellation-ownership checks.
• Preserve named-pipe telemetry delivery, reconnect, mount-error, and connection-timeout checks.
• Keep isolated policy, parser, controlled-stream, and filesystem-only libgit2 tests in the unit suite.
• Keep native hook executable tests and deterministic in-process Windows API tests in the unit suite.
• Reuse shared test support and isolate configuration and environment state in the migrated fixtures.
• Resolve Git through the product platform API. Use that installation for Bash and every local Git fixture.
• Resolve the mount executable beside the selected payload.
• Run local component fixtures through the existing functional runner and service lifecycle in PR-build and release-build jobs.
• Reject empty selections, unexpected tests, and skipped local component cases in the targeted CI entry point.
• Document the local component exception to the black-box functional-test convention. These fixtures call product APIs without a mounted enlistment or remote repository.