fix(network): clamp max_inflight_block_bytes up to the checkpoint floor

[evan-forbes]

Motivation

#292 enforces a checkpoint-safe block apply and byte budget floor. Older Zakura configs can still contain a positive zakura.block_sync.max_inflight_block_bytes below that checkpoint-range floor, which would make zebrad reject the config at startup.

Solution

Clamp positive below-floor max_inflight_block_bytes values up to BS_CHECKPOINT_RANGE_BYTE_FLOOR during config deserialization, with a warning. Zero remains invalid and is still rejected by validation.

This PR is intentionally stacked on #292 and contains only the config compatibility clamp.

Tests

Passed locally on codex/block-sync-checkpoint-byte-clamp:

• cargo fmt --all -- --check
• cargo test -p zebra-network block_sync_config_defaults_and_round_trips --lib
• cargo test -p zebra-network config_ --lib (12 passed, including config_clamps_below_floor_inflight_block_bytes, config_deserialize_clamps_below_floor_inflight_block_bytes, and zero-invalid validation coverage)

AI Disclosure

• AI tools were used: Codex was used to split the change from fix(network): enforce checkpoint-safe block apply budget #292, run focused validation, and draft this PR description.

PR Checklist

• The PR title follows conventional commits format: type(scope): description
• The PR follows the contribution guidelines.
• The solution is tested as described above.