[FA] Read name from UPDATER_TASK

[coignetp]

What does this PR do?

Read the target DatadogAgent namespace/name from the UPDATER_TASK params directly (via a new parseTaskNSN helper) for start, stop, and promote operations, instead of storing it in a experimentTarget field on the daemon struct.

Motivation

Previously, stop and promote operations relied on d.experimentTarget being set by a prior start call. This stateful approach was fragile — if the daemon restarted between start and stop/promote, the target was lost. Carrying the name in every task payload makes each operation self-contained and idempotent.

Additional Notes

• Removes the experimentTarget types.NamespacedName field from the daemon struct.
• NamespacedName is now a field of experimentParams (carried in the task itself).
• validateOperation tests for empty name/namespace are replaced by parseTaskNSN tests.

Minimum Agent Versions

No minimum version requirements.

Describe your test plan

Unit tests in pkg/fleet/daemon_test.go and pkg/fleet/remote_config_test.go are updated to pass NamespacedName inside experimentParams. New TestParseTaskNSN_* tests cover the valid, missing-name, and missing-namespace cases.

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)

[datadog-official]

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🛑 Gate Violations

🎯 1 Code Coverage issue detected

A Patch coverage percentage gate may be blocking this PR.

• Patch coverage: 63.16% (threshold: 80.00%)

ℹ️ Info

🎯 Code Coverage (details)
• Patch Coverage: 63.16%
• Overall Coverage: 40.57%

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 65.00000% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 40.50%. Comparing base (a816a2a) to head (f3aad2b).

Files with missing lines	Patch %	Lines
pkg/fleet/daemon.go	46.15%	3 Missing and 4 partials ⚠️

Additional details and impacted files

@@                      Coverage Diff                       @@
##           khewonc/experiment-signals    #2913      +/-   ##
==============================================================
- Coverage                       40.54%   40.50%   -0.04%     
==============================================================
  Files                             321      320       -1     
  Lines                           28514    28395     -119     
==============================================================
- Hits                            11560    11501      -59     
+ Misses                          16107    16048      -59     
+ Partials                          847      846       -1     

Flag	Coverage Δ
unittests	40.50% <65.00%> (-0.04%)	⬇️

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

Files with missing lines	Coverage Δ
pkg/fleet/experiment.go	84.61% <100.00%> (+0.94%)	⬆️
pkg/fleet/remote_config.go	100.00% <ø> (ø)
pkg/fleet/daemon.go	64.98% <46.15%> (-0.48%)	⬇️

... and 4 files with indirect coverage changes

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