Fix MHQ 8679: failure to load saves

[Sleet01]

This fixes an issue where some save game files - including autosaves - could not be loaded at all due to XStream stupidity regarding Records.
Huge props to @BTSS88 for tracking down the root cause: auto-hit artillery markers (or to be more precise, the BoardLocation data storing those markers' locations, which used to be a class but got refactored to a Record)

XStream 1.4, the version we currently depend on, does not natively support deserializing Records.
This is deeply stupid, but made even more offensive by the fact that it can serialize them just fine.

So every time we add a Record data type that is Serializable, we have to also add a new Converter in SerializationHelper.java to ensure that this data type can be loaded after saving it.
This was not done when BoardLocation was converted to a Record, but it turns out XStream doesn't care if there are no actual BoardLocation entries populated in the save game file so few players noticed.

This patch adds the missing Converter, as well as making sure to call context.convertAnother() so that any Coords created as part of the BoardLocation load are also registered with proper ID references so that later things like the special hex HashTable don't explode.

Testing:

• Loaded the OP failing save game and played through some turns
• Ran all 3 projects' unit tests

Fix MegaMek/mekhq#8679

[codecov]

Codecov Report

❌ Patch coverage is 0% with 22 lines in your changes missing coverage. Please review.
✅ Project coverage is 29.84%. Comparing base (fe0b8b7) to head (03cb567).
⚠️ Report is 83 commits behind head on main.

Files with missing lines	Patch %	Lines
...k/src/megamek/common/util/SerializationHelper.java	0.00%	22 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #7949      +/-   ##
============================================
+ Coverage     29.77%   29.84%   +0.06%     
- Complexity    16655    16831     +176     
============================================
  Files          3134     3140       +6     
  Lines        301874   302533     +659     
  Branches      52863    52988     +125     
============================================
+ Hits          89895    90300     +405     
- Misses       202646   202810     +164     
- Partials       9333     9423      +90     

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[Copilot]

Pull request overview

This pull request fixes a critical bug where save game files (including autosaves) could not be loaded due to XStream 1.4's inability to deserialize Java Records. The issue was introduced when BoardLocation was refactored from a class to a Record without adding the necessary XStream converter.

Changes:

• Added a custom XStream converter for BoardLocation Records in SerializationHelper.java to enable proper deserialization
• Used context.convertAnother() to ensure Coords objects are properly registered in XStream's reference tracking system
• Accidentally added an unused import to WeaponHandler.java

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
megamek/src/megamek/common/weapons/handlers/WeaponHandler.java	Adds an unused AbstractReflectionConverter import
megamek/src/megamek/common/util/SerializationHelper.java	Implements a custom converter for BoardLocation Records to fix save game deserialization

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