Expose channel_reserve_satoshis via ChannelParameters

[yashbhutwala]

Summary

• Add channel_reserve_satoshis: Option<u64> field to ChannelParameters struct
• For V1 channels: Returns Some(value) with the explicit reserve from open_channel message
• For V2 channels: Returns None since reserve is calculated from total channel value (initiator + acceptor funding), which isn't known at OpenChannelRequest time
• Move channel_parameters() from CommonOpenChannelFields to individual OpenChannel and OpenChannelV2 impls to handle V1/V2 difference correctly
• Add test test_open_channel_request_channel_reserve_satoshis to verify the implementation

Fixes #3909

Revives the closed PR #3910 with the approach suggested by reviewers:

• Uses Option<u64> instead of trying to read from wire for V2 (which doesn't have this field)
• Follows @wpaulino's suggestion to move channel_parameters() to OpenChannel/OpenChannelV2
• Properly documents the V1 vs V2 behavior

Test plan

• cargo check -p lightning passes
• All 1191 lightning tests pass (cargo test -p lightning --lib)
• New test test_open_channel_request_channel_reserve_satoshis validates V1 channels correctly populate the field
• All 41 channel_open_tests pass

🤖 Generated with Claude Code

[ldk-reviews-bot]

👋 Thanks for assigning @tankyleo as a reviewer!
I'll wait for their review and will help manage the review process.
Once they submit their review, I'll check if a second reviewer would be helpful.

[codecov]

Codecov Report

❌ Patch coverage is 51.72414% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.60%. Comparing base (4281e49) to head (2f585af).

Files with missing lines	Patch %	Lines
lightning/src/ln/msgs.rs	50.00%	13 Missing ⚠️
lightning/src/ln/channelmanager.rs	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4319      +/-   ##
==========================================
- Coverage   86.61%   86.60%   -0.01%     
==========================================
  Files         158      158              
  Lines      102730   102748      +18     
  Branches   102730   102748      +18     
==========================================
+ Hits        88977    88989      +12     
- Misses      11335    11342       +7     
+ Partials     2418     2417       -1     

Flag	Coverage Δ
fuzzing	36.08% <51.72%> (-1.01%)	⬇️
tests	85.90% <51.72%> (-0.01%)	⬇️

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