Optimize E2E workflow: build ToolHive binary only once

[JAORMX]

Summary

Optimizes the E2E tests workflow by building the ToolHive binary only once and reusing it across all matrix jobs, significantly reducing CI execution time.

Changes

• Added build-binary job: Creates a separate job that builds the ToolHive binary once using the existing build process
• Binary artifact management: Uploads the built binary as an artifact with 1-day retention
• Job dependency: Modified e2e-tests job to depend on build-binary via needs: build-binary
• Eliminated redundant builds: Removed the "Build ToolHive binary" step from each matrix job and replaced with artifact download
• Maintained necessary dependencies: Kept Go setup and Ginkgo CLI installation in test jobs since they're still needed

Benefits

• Reduced build time: Instead of building the binary 3 times (once per matrix job), it's now built only once
• Parallel execution: All three matrix jobs (core, mcp, proxy-mw) can now run in parallel after the single build completes
• Resource optimization: Less CPU and memory usage in the CI pipeline
• Maintained test coverage: No changes to actual test execution or coverage

Workflow Structure

Before: e2e-tests (3 parallel jobs, each building binary)
After: build-binary → e2e-tests (3 parallel jobs using shared binary)

Testing

The workflow maintains the same test execution pattern and environment variables. The only change is how the binary is obtained (artifact download vs. local build).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 46.69%. Comparing base (68db7a6) to head (7bcd256).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1937   +/-   ##
=======================================
  Coverage   46.69%   46.69%           
=======================================
  Files         220      220           
  Lines       27378    27378           
=======================================
  Hits        12785    12785           
  Misses      13620    13620           
  Partials      973      973           

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