Unify e2e yardstick backend on 1.2.0

[jhrozek]

Follow-up to #5981 and #5993.

Summary

Why: #5981 temporarily split the e2e yardstick backend image — Legacy 1.1.1 for the operator/vMCP suites, Modern 1.2.0 only for the dual-era tests — because the vMCP classified a session-mode go-sdk v1.7 backend as Modern (from a server/discover 200) and then could not drive it (Modern data-plane calls return errWrongEra). #5993 fixed that: the vMCP now keys revision classification on server/discover's supportedVersions, so a session-mode v1.7 backend — whose supportedVersions omits 2026-07-28 — is correctly classified Legacy and driven via the Legacy data plane. With #5993 in main, the split is no longer needed.

What: collapse back to a single images.YardstickServerImage at 1.2.0 for every e2e suite, drop the temporary YardstickServerImageDualEra constant, and unify the workflow image pulls/loads (net −11 lines).

Type of change

• Other (describe): Test-infra only — e2e image constant + CI workflow image references. No production code, no test logic change.

Test plan

• Unit tests (task test): TestProbeRevision_RealBackends (the Support MCP 2026-07-28 spec (go-sdk v1.7 via toolhive-core) #5993 anti-regression pin — a real session-mode go-sdk v1.7 backend must classify Legacy) passes on main, confirming a 1.2.0 backend in session mode is driven Legacy by the vMCP.
• E2E tests (task test-e2e): the dual-era suite passes locally on the unified 1.2.0 image (11/11). The vMCP/virtualmcp/operator suites on 1.2.0 are gated by this PR's CI (E2E Tests Core (vmcp), E2E Test Lifecycle).

Does this introduce a user-facing change?

No.

Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.05%. Comparing base (ff0ba2e) to head (8ed3178).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6004      +/-   ##
==========================================
- Coverage   72.05%   72.05%   -0.01%     
==========================================
  Files         720      720              
  Lines       74641    74680      +39     
==========================================
+ Hits        53786    53814      +28     
- Misses      17001    17012      +11     
  Partials     3854     3854              

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