[FA] Serialize UPDATER_TASK execution and fix no-op config version clearing

[coignetp]

What does this PR do?

Ports three fixes from #2952 that are independent of the parseTaskNSN / namespace-from-task refactor.

1. Serialize UPDATER_TASK execution (taskMu + handleTask)

Adds a taskMu sync.Mutex to Daemon and extracts a handleTask method that holds the lock for the full lifecycle of one task (RUNNING → DONE/ERROR/INVALID_STATE). The inline block in Start() is replaced with return d.handleTask(ctx, req).

Why: Multiple concurrent RC callbacks can race on setTaskState. This matches the datadog-agent's single-writer model.

2. No-op stop clears experiment config version

When canStop returns isNoOp=true (e.g. spec already rolled back due to timeout), the experiment config version is now cleared before returning.

Why: Without this, the backend could not issue new tasks after a timeout-triggered rollback.

3. No-op promote clears experiment config version

Same fix for promoteDatadogAgentExperiment when the experiment is already in a terminal state.

Checklist

• PR has at least one valid label: bug, enhancement, refactoring, documentation, tooling, and/or dependencies
• PR has a milestone or the qa/skip-qa label
• All commits are signed (see: signing commits)

[codecov-commenter]

Codecov Report

❌ Patch coverage is 94.73684% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 41.45%. Comparing base (099f33f) to head (17d9deb).

Files with missing lines	Patch %	Lines
pkg/fleet/daemon.go	94.73%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2956      +/-   ##
==========================================
+ Coverage   41.38%   41.45%   +0.07%     
==========================================
  Files         327      327              
  Lines       28952    28961       +9     
==========================================
+ Hits        11982    12006      +24     
+ Misses      16109    16094      -15     
  Partials      861      861              

Flag	Coverage Δ
unittests	41.45% <94.73%> (+0.07%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
pkg/fleet/daemon.go	71.83% <94.73%> (+6.37%)	⬆️

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[datadog-prod-us1-6]

🎯 Code Coverage (details)
• Patch Coverage: 94.44%
• Overall Coverage: 41.59% (+0.07%)

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[khewonc]

Incorporated in #2944. Closing in favor of that PR