Fix passing through misc keys in encode-mtg2

[pgeier]

Description

All keys with "misc-" prefix never made it properly to the mars2grib.

Contributor Declaration

By opening this pull request, I affirm the following:

• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.

🌈🌦️📖🚧 Documentation 🚧📖🌦️🌈
https://sites.ecmwf.int/docs/dev-section/multio/pull-requests/PR-275

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 33.21%. Comparing base (d3e61e9) to head (7d9b19a).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #275      +/-   ##
===========================================
- Coverage    33.21%   33.21%   -0.01%     
===========================================
  Files          374      374              
  Lines        41063    41061       -2     
  Branches      1701     1703       +2     
===========================================
- Hits         13638    13637       -1     
+ Misses       27425    27424       -1     

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[Copilot]

Pull request overview

This PR fixes forwarding of misc-* metadata from MultIO’s encode-mtg2 action into metkit’s mars2grib encoder by dumping the misc record without its misc- key prefix, ensuring those keys are interpreted correctly by mars2grib.

Changes:

• Switch misc configuration generation from dumpRecord(...) to dumpUnscopedRecord(...) so misc-* keys arrive at mars2grib with the expected key names.
• Remove unused (mapped) mars/misc LocalConfiguration dumps in executeImpl.
• Remove a stale TODO related to writing mapped metadata into the output message header.

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