fix(gmp): fix keeper initialization and packet unmarshaling

[mariuszzak]

Description

Fix two critical bugs in ICS27-GMP module:

1. keeper: Initialize msgRouter and accountKeeper fields that were being passed to NewKeeper but not assigned to the keeper struct, causing nil pointer panics when routing messages

2. packet: Fix UnmarshalPacketData to use correct Go unmarshal pattern by initializing pointer before passing to json.Unmarshal instead of passing pointer-to-pointer

---

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• Linked to GitHub issue with discussion and accepted design, OR link to spec that describes this work.
• Include changelog entry when appropriate (e.g. chores should be omitted from changelog).
• Wrote unit and integration tests if relevant.
• Updated documentation (docs/) if anything is changed.
• Added godoc comments if relevant.
• Self-reviewed Files changed in the GitHub PR explorer.
• Provide a conventional commit message to follow the repository standards.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 7 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (serdar/xxx-contract-calls@be8367e). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
modules/apps/27-gmp/keeper/keeper.go	0.00%	5 Missing ⚠️
modules/apps/27-gmp/types/packet.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@                     Coverage Diff                      @@
##             serdar/xxx-contract-calls    #8660   +/-   ##
============================================================
  Coverage                             ?   56.37%           
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  Files                                ?      335           
  Lines                                ?    23605           
  Branches                             ?        0           
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  Hits                                 ?    13307           
  Misses                               ?     9691           
  Partials                             ?      607           

Flag	Coverage Δ
08-wasm	65.99% <ø> (?)
e2e	1.14% <ø> (?)
ibc-go	61.35% <0.00%> (?)

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[srdtrk]

Great catch.