rebalance: support custom CLN version strings

[bittylicious]

Summary

• include the upstream channel lookup fix from rebalance: handle channels without short_channel_id lightningd/plugins#888 because the fork master predates it
• tolerate custom non-semver CLN build strings by validating required RPC commands
• detect legacy and modern getroutes path schemas from returned fields instead of the version string
• add regression coverage for custom and short versions, plugin initialization, and both route formats

Background

A custom CLN build reports opencode/spenderp-defer-awaiting-recovery from getinfo.version. The plugin treated that value as a numeric release and failed during initialization before setting its runtime state.

Testing

• pytest -q rebalance/test_clnutils.py (11 passed)
• ruff check and ruff format --check
• python3 -m py_compile
• git diff --check

Draft for review and live compatibility validation before considering an upstream submission.

[bittylicious]

Superseded by the reduced upstream draft lightningd#891.