Improve CI workflow ordering

[corymhall]

Summary

• split provider unit tests and schema comparison out of the serialized prerequisites job
• keep prerequisites focused on lightweight artifact prep so provider/SDK builds can start sooner
• restore Go cache in example test shards to reduce repeated package compile time

Context

This revives the useful workflow-ordering shape from the earlier closed experiment in #5785 while keeping the changes scoped to the current branch. The current ci-mgmt generator does not yet model this fan-out, so the generated workflow edits are intentionally manual for this timing experiment.

Verification

• mise exec -- make ci-mgmt
• mise exec -- make schema_embed
• make -n schema_embed
• make -n test_provider
• YAML parse for touched workflows
• actionlint -ignore 'label \"pulumi-ubuntu-8core\" is unknown' .github/workflows/prerequisites.yml .github/workflows/run-acceptance-tests.yml .github/workflows/test.yml
• git diff --check

[github-actions]

Does the PR have any schema changes?

Looking good! No breaking changes found.
No new resources/functions.

Maintainer note: consult the runbook for dealing with any breaking changes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 14.48%. Comparing base (46548bf) to head (5219613).
⚠️ Report is 15 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6366   +/-   ##
=======================================
  Coverage   14.48%   14.48%           
=======================================
  Files         374      374           
  Lines       89963    89963           
=======================================
  Hits        13032    13032           
  Misses      76909    76909           
  Partials       22       22           

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