esplora: don't panic in raw_to_num on bad response

[vincenzopalazzo]

Summary

• raw_to_num previously .expect()-ed both the UTF-8 decode and the i64::parse, which crashed the bitcoin backend whenever an esplora endpoint returned anything non-numeric (HTML error page, captive portal interstitial, mangled body through a proxy, etc.). The panic message was "impossible parse a string into a i64: ParseIntError { kind: InvalidDigit }" and took the whole node down.
• Change the function to return Result<i64, PluginError> and surface the offending bytes in the error, so RecoveryStrategy::apply can retry instead of crashing.
• Trim whitespace before parsing as a small robustness improvement, and log the parsed value at debug for diagnostics.
• Updated the two .map(|raw| raw_to_num(&raw)) callers in sync_block_by_height and sync_chain_info to .and_then(...) to match the new signature.

Test plan

• cargo build --release -p folgore-esplora -p folgore_plugin succeeds with no warnings.
• Reproduced the original panic on a node that hit the bug, then verified the patched plugin recovers cleanly — bitcoin backend stays alive and raw_to_num input: "..." (len=N) debug logs show clean integer bodies on each /blocks/tip/height poll.
• Node getinfo returns healthy state and the daemon resumes block sync after the patch.

🤖 Generated with Claude Code

Fixes #100