Test CI on Ubuntu 26.04

[danbarr]

Summary

• GitHub will move ubuntu-latest to Ubuntu 26.04; this draft validates the pull-request CI paths against that image before the alias changes.
• Pin the PR-reachable standard-runner jobs to ubuntu-26.04, including six E2E shards and MCP conformance.

Type of change

• Other (temporary CI validation)

Test plan

• Linting (task lint)
• Manual testing: reviewed the workflow diff and verified it preserves the large-runner and slim-runner selections.
• E2E tests (task test-e2e)

Changes

File group	Change
PR validation and scan workflows	Pin standard jobs to Ubuntu 26.04.
E2E workflow	Pin standard-runner matrix shards and MCP conformance to Ubuntu 26.04; preserve the large-runner fallback.

Does this introduce a user-facing change?

No.

Special notes for reviewers

This is intentionally a draft validation PR and must not be merged. It excludes main/release-only workflows because a PR cannot safely exercise their publishing paths.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.42%. Comparing base (d7877b8) to head (22185b9).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6769      +/-   ##
==========================================
+ Coverage   79.41%   79.42%   +0.01%     
==========================================
  Files         806      808       +2     
  Lines       81969    82060      +91     
==========================================
+ Hits        65096    65179      +83     
- Misses      16868    16876       +8     
  Partials        5        5              

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