fix: report function selector on estimation failure

[rubydusa]

Closes #22.

Summary

GasKillerReport::report_error previously hard-coded function_selector to FixedBytes::default() (0x00000000). On the success path the selector was correctly extracted from the transaction's input, but every error report carried zero — so a failed estimation looked indistinguishable from a selector-less call, and there was no way to tell which method triggered the failure.

The fix lifts the selector lookup into get_report so it's available on both branches:

• get_report calls a new helper fetch_function_selector(provider, tx_hash) once at the top.
• The selector is threaded into gaskiller_reporter (replacing the in-place lookup there) and into report_error on the failure path.
• The outer fallback in gas_estimate_block is only reachable now if the transaction fetch itself fails, in which case there's no selector to report and we fall back to FixedBytes::default().

The in-gaskiller_reporter get_transaction_by_hash call is removed since the caller now supplies the selector — net no extra RPC roundtrips.

Behavioural notes

• Success reports: unchanged.
• Error reports (estimation failure): now carry the real selector instead of zero.
• Plain ETH transfers / sub-4-byte inputs: previously erroring inside gaskiller_reporter with \"could not get function selector\", now degrade gracefully — function_selector() returns None, we use unwrap_or_default() (zero), and the rest of the pipeline runs. Upstream filters in gas_estimate_tx (smart-contract check) and gas_estimate_block (to.is_some()) already exclude these in the common case, so this should mostly be defence-in-depth.
• Single-tx CLI path (gas_estimate_tx): if get_transaction_by_hash itself fails (network error, missing tx), get_report now propagates Err instead of silently producing a zero-selector error report. The CLI already handled top-level Err via ?.

Verification

• cargo test -p gas-analyzer-anvil --lib — 11/11 pass locally (against Sepolia via RPC_URL, Foundry v1.5.1).
• cargo test (default workspace, EvmSketch path) — pass.
• cargo clippy -p gas-analyzer-anvil --all-targets -- -D warnings — clean.
• cargo clippy --workspace --exclude gas-killer-wasm --exclude gas-analyzer-anvil --all-targets -- -D warnings — clean (matches CI lint).
• cargo fmt --all -- --check — clean.

🤖 Generated with Claude Code

[rubydusa]

ci is failing because of new foundry release: see
#96 (comment)

[rubydusa]

put on draft as long as #96 isn't merged