Add daemon to the user agent when the CLI is started in daemon mode

[MatteoPologruto]

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• Tests for the changes have been added (for bug fixes / features)
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What kind of change does this PR introduce?

Bug fix

What is the current behavior?

The user agent does not specify if the CLI runs in daemon mode. It is always something like arduino-cli/git-snapshot (amd64; windows; go1.23.2) Commit:84fc413a.

What is the new behavior?

network.user_agent_ext is set too daemon when the CLI runs in daemon mode. The complete user agent is arduino-cli/git-snapshot daemon (amd64; windows; go1.23.2) Commit:84fc413a.

Does this PR introduce a breaking change, and is titled accordingly?

Other information

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 5 lines in your changes missing coverage. Please review.

Project coverage is 67.55%. Comparing base (84fc413) to head (1dc53f7).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
internal/cli/daemon/daemon.go	50.00%	4 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2789      +/-   ##
==========================================
- Coverage   67.57%   67.55%   -0.03%     
==========================================
  Files         238      238              
  Lines       22362    22372      +10     
==========================================
+ Hits        15111    15113       +2     
- Misses       6062     6068       +6     
- Partials     1189     1191       +2     

Flag	Coverage Δ
unit	67.55% <50.00%> (-0.03%)	⬇️

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[alessio-perugini]

Left some comments/
It would be cool if we're able to test it.

Maybe we could use a proxy HTTP, and inspect the call that arrives are with the expected User agent. Something similar to: https://github.com/arduino/arduino-cli/blob/master/internal/cli/configuration/network_test.go#L29-L51

[alessio-perugini]

What should we do in case this value is not empty?

1. Should we always override with the daemon value
2. should we append to the previous value
3. should we honor the user setting?

/cc @cmaglie