PMM-7 Skip PMM agent setup if already configured

[BupycHuk]

Added a check to determine if the PMM agent is already configured by verifying the presence of an existing configuration. If the agent is configured, the setup process is skipped, preventing unnecessary reconfiguration. This optimizes startup behavior and avoids redundant setup operations.

PMM-7

Link to the Feature Build: SUBMODULES-3941

If this PR adds or removes or alters one or more API endpoints, please review and add or update the relevant API documents as well:

• API Docs updated

If this PR is related to some other PRs in this or other repositories, please provide links to those PRs:

• Links to related pull requests (optional).

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 40 lines in your changes missing coverage. Please review.

Project coverage is 43.58%. Comparing base (f48899c) to head (5bc2651).

Files with missing lines	Patch %	Lines
agent/cmd/pmm-agent-entrypoint/main.go	0.00%	40 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##               v3    #4051      +/-   ##
==========================================
- Coverage   43.60%   43.58%   -0.03%     
==========================================
  Files         369      369              
  Lines       45225    45246      +21     
==========================================
  Hits        19719    19719              
- Misses      23798    23819      +21     
  Partials     1708     1708              

Flag	Coverage Δ
agent	52.51% <0.00%> (-0.10%)	⬇️

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