feat: add governance proposal methods to UniFFI bridge

[xdustinface]

Summary

• Add get_governance_proposals() method returning Vec
• Add get_governance_proposal(hash) for single proposal lookup
• Stub implementations (return empty data) since governance engine not yet in SPV client
• Bridge tests for new methods

Closes #38

Test plan

• get_governance_proposals returns empty vec (stub)
• get_governance_proposal returns None for unknown hash

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.54%. Comparing base (6adcc57) to head (90d5fdb).
⚠️ Report is 1 commits behind head on v0.42-dev.
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Additional details and impacted files

@@              Coverage Diff              @@
##           v0.42-dev      #39      +/-   ##
=============================================
- Coverage      72.80%   67.54%   -5.27%     
=============================================
  Files            287      316      +29     
  Lines          57461    67696   +10235     
=============================================
+ Hits           41836    45726    +3890     
- Misses         15625    21970    +6345     

Flag	Coverage Δ
core	75.14% <ø> (ø)
ffi	37.82% <ø> (?)
rpc	19.92% <ø> (ø)
spv	84.25% <100.00%> (+0.04%)	⬆️
wallet	66.40% <ø> (ø)

Files with missing lines	Coverage Δ
dash-spv/src/bridge/mod.rs	89.81% <100.00%> (+0.32%)	⬆️

... and 30 files with indirect coverage changes

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