Add tests for multi_parameter and multi_function methods

[Copilot]

Adds test coverage for the new multi_parameter and multi_function methods in ChannelTuple as requested in #7686.

Tests added

• multi_parameter: Valid parameter lookup returns MultiChannelInstrumentParameter, invalid name raises AttributeError, empty channel tuple raises AttributeError
• multi_function: Valid function lookup returns callable, callable invokes function on all channels, works with method arguments, invalid name raises AttributeError

Example usage

# Type-safe parameter access
temp_param = dci.channels.multi_parameter("temperature")
temperatures = temp_param.get()  # Returns tuple of all channel temperatures

# Type-safe function access  
multi_func = dci.channels.multi_function("log_my_name")
multi_func()  # Calls log_my_name on all channels

• Short description included
• Automatic tests added
• Changelog entry (not needed - tests only)

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.79%. Comparing base (2b33205) to head (6c44af9).
⚠️ Report is 3 commits behind head on jenshnielsen/add_channel_type.

Additional details and impacted files

@@                        Coverage Diff                        @@
##           jenshnielsen/add_channel_type    #7687      +/-   ##
=================================================================
+ Coverage                          59.74%   59.79%   +0.05%     
=================================================================
  Files                                352      352              
  Lines                              31681    31681              
=================================================================
+ Hits                               18927    18944      +17     
+ Misses                             12754    12737      -17     

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