Retry dual-era backend startup on a lost port-bind race

[jhrozek]

Summary

• Why: test/e2e/vmcp_dual_era_test.go's BeforeEach intermittently fails
  waiting for a yardstick backend to become running, with the container
  erroring out as driver failed programming external connectivity ... bind: address already in use. allocateVMCPPort (vmcp_cli_helpers_test.go)
  probes for a free port and closes its own listener immediately, but the
  actual bind happens later, asynchronously, inside the container Docker
  starts for the detached thv run process — under back-to-back workload
  churn, another container can grab the same ephemeral port first. This is a
  classic check-then-use race across process boundaries; the only way to
  make progress in the face of it is to retry, not to check earlier/later.
• What: startYardstickLegacyOnPort/startYardstickModernOnPort now go
  through a shared startEraBackendOnPort helper that retries once with a
  freshly allocated port if WaitForMCPServer fails with
  "address already in use".

Type of change

• Bug fix

Test plan

• E2E tests (task test-e2e)
• Manual testing (describe below)

Ran ginkgo --label-filter='vmcp && dual-era' --repeat=20 -v locally against
this fix. All 20 repeats (140 specs) passed. The port race fired once during
the run (visible in the log as lost a port-bind race on <port>, retrying with a fresh port) and was absorbed by the retry instead of failing the
spec — confirming both that the race is real and that the fix works.

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. Test-only change.

Special notes for reviewers

This fix is scoped to the two call sites that actually race Docker's async
container bind. allocateVMCPPort is also used elsewhere for thv vmcp serve's own port, which is bound synchronously in-process by the same test
binary and isn't subject to this race, so I left allocateVMCPPort itself
unchanged rather than touching its other callers.

Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.24%. Comparing base (e8f84ef) to head (1a0802c).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6080      +/-   ##
==========================================
- Coverage   72.26%   72.24%   -0.03%     
==========================================
  Files         728      728              
  Lines       75523    75523              
==========================================
- Hits        54580    54561      -19     
- Misses      17040    17057      +17     
- Partials     3903     3905       +2     

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