fix(session): preserve halved log scan ranges

[EfeDurmaz16]

Summary

• keep both halves of an oversized eth_getLogs block range when the RPC asks for a smaller query
• process pending split ranges before advancing below the original chunk, so the upper half is not silently skipped
• add unit coverage for the split range boundaries

Closes #412.

Verification

• PATH="$HOME/.cargo/bin:$PATH" cargo +nightly fmt --all -- --check
• RUSTC="$(rustup which --toolchain stable rustc)" rustup run stable cargo test -p tempo-common payment::session::channel::tests
• RUSTC="$(rustup which --toolchain stable rustc)" rustup run stable cargo test -p tempo-common
• RUSTC="$(rustup which --toolchain nightly rustc)" PATH="$HOME/.cargo/bin:$PATH" cargo +nightly clippy -p tempo-common --all-targets --all-features --locked -- -D warnings