feat: pause DDL during TiDB version upgrade (smooth upgrade)

[tennix]

Summary

• Introduces /upgrade/start and /upgrade/finish lifecycle hooks in the TiDBGroup reconciliation loop, mirroring TiUP's smooth upgrade behavior
• TaskSmoothUpgradeStart runs before TaskUpdater to pause DDL; TaskSmoothUpgradeFinish runs after TaskStatusRevisionAndReplicas to resume DDL
• A persistent annotation (tidb.core.pingcap.com/smooth-upgrade-phase: in-progress) tracks in-flight state across operator restarts

Behavior

The keyspace value from spec.template.spec.keyspace is passed directly to /upgrade/start.

Guards:

• Only fires when spec.version changes (not scale in/out or config changes)
• Both source and target versions must be ≥ v7.5.0
• If /upgrade/start fails, rolling upgrade is blocked (retried) — no pods restart without a successful DDL pause

Note: Dedicated deployments on release-1.x (TiDB Operator v1) require a separate implementation and PR on that branch.

Changes

• api/core/v1alpha1/tidb_types.go — annotation key/value constants
• pkg/tidbapi/v1/types.go — UpgradeRequest struct
• pkg/tidbapi/v1/client.go — UpgradeStart + UpgradeFinish on TiDBClient interface
• pkg/compatibility/semver.go — SupportsSmoothUpgrade(version) bool
• pkg/controllers/tidbgroup/tasks/upgrade.go — new TaskSmoothUpgradeStart + TaskSmoothUpgradeFinish
• pkg/controllers/tidbgroup/builder.go — wire new tasks into reconciliation loop

Test plan

• make test passes (unit tests for new tasks, HTTP client methods, semver helper)
• Version upgrade v7.5.x → v8.x: verify annotation appears before first pod restarts, removed after last pod upgraded
• Scale-only change: verify annotation is never set
• Upgrade from v7.4.x: verify smooth upgrade is skipped
• Premium keyspace upgrade: verify /upgrade/start body contains keyspace name

🤖 Generated with Claude Code

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 76.40449% with 21 lines in your changes missing coverage. Please review.
✅ Project coverage is 37.77%. Comparing base (2b81667) to head (dd778a7).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6892      +/-   ##
==========================================
+ Coverage   37.44%   37.77%   +0.33%     
==========================================
  Files         392      393       +1     
  Lines       22432    22572     +140     
==========================================
+ Hits         8399     8526     +127     
- Misses      14033    14046      +13     

Flag	Coverage Δ
unittest	37.77% <76.40%> (+0.33%)	⬆️

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