macos_online_fix

[mickaelbegon]

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This PR enables online_optim on macOS by switching OnlineOptim.DEFAULT to MULTIPROCESS_SERVER instead of disabling online plotting entirely.

It also fixes the supporting issues that showed up during macOS validation:

• recreate the TCP client socket after failed connect attempts
• auto-pick a free port for the auto-started plotting server
• keep MULTIPROCESS explicitly unsupported on macOS
• skip the final basic_ocp animation cleanly when bioviz is not installed
• ignore all-NaN line data when computing online plot bounds

I split the change into two commits:

1. c9811420 Enable macOS online plotting via multiprocess server
2. 2b17fb3e Harden macOS plotting example fallbacks

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This change is 

[codecov]

Codecov Report

❌ Patch coverage is 78.12500% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.21%. Comparing base (e8723c9) to head (a01ed91).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
bioptim/gui/online_callback_server.py	60.00%	4 Missing ⚠️
bioptim/gui/online_callback_multiprocess_server.py	70.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1047      +/-   ##
==========================================
+ Coverage   77.00%   77.21%   +0.21%     
==========================================
  Files         192      192              
  Lines       21127    21148      +21     
==========================================
+ Hits        16269    16330      +61     
+ Misses       4858     4818      -40     

Flag	Coverage Δ
unittests	77.21% <78.12%> (+0.21%)	⬆️

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[pariterre]

@pariterre reviewed 10 files and all commit messages, and made 2 comments.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on mickaelbegon).

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bioptim/examples/getting_started/basic_ocp.py line 192 at r2 (raw file):

            print("Animation skipped because bioviz is not installed.")
        else:
            raise

I don't understand the benefit of this check as it already fails like so when bioviz is not installed.. I think commenting is better...
(Changes are already done)

[pariterre]

@pariterre reviewed 1 file and all commit messages, and made 1 comment.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on mickaelbegon).