fix(lading): surface remote-input parsing failures as errors

[goxberry]

Summary

Second PR in the FIXME-conversion stack. Addresses two sites where
malformed remote input — the Splunk HEC ack-id response and the
Prometheus scrape body — caused lading to crash.

generator/splunk_hec.rs::send_hec_request

Add Error::ResponseParse(#[from] serde_json::Error) and propagate
the JSON parse failure with ? instead of .expect(). Drop the
fn-level #[expect]. A malformed ack-id body is now a recoverable
generator error rather than a panic.

target_metrics/prometheus.rs::parse_prometheus_metrics

This is an ad-hoc Prometheus parser called for side effects from a
scrape loop. Returning a Result would require redesigning the
caller; the FIXME asked for recoverable errors, so we recover at
the line granularity: each former .expect() becomes a
let Some(...) else { warn!; continue; } (or a match for the
MetricType FromStr parse), so malformed lines are logged and
skipped rather than crashing the scraper. The two regex-capture
sites inside the label .map(|cap| ..) become .filter_map(|cap| ..)
with the same warn-and-skip pattern.

8 former .expect() sites in this function are now non-fatal log
points. The fn-level #[expect] is removed.

Test plan

• ./ci/validate passes
• grep -n 'FIXME' lading/src/generator/splunk_hec.rs lading/src/target_metrics/prometheus.rs is empty
• cargo build --workspace --all-targets --all-features

🤖 Generated with Claude Code

[goxberry]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• fix(lading): surface tooling/FUSE-mount panics as errors #1904
• fix(lading): surface remote-input parsing failures as errors #1903 👈 (View in Graphite)
• fix(lading): surface user-config parsing failures as Error variants #1902
• refactor(lading): drop expect_used quarantines and annotate bins #1901
• refactor(lading): annotate intentional-panic observer/target sites #1900
• refactor(lading): annotate intentional-panic network generators #1899
• refactor(lading): annotate intentional-panic file_gen sites #1898
• refactor(lading): annotate intentional-panic blackhole + small generator sites #1897
• refactor(lading): convert structural-infallible .expect() to unreachable!() #1896
• refactor(capture): retire .expect() sites and drop quarantines #1894
• refactor(payload): retire remaining .expect() sites and drop quarantine #1893
• fix(payload): reject zero-byte arbitrary input for Block #1892
• refactor(payload): annotate intentional-panic .expect() sites #1891
• refactor(payload): mark in-function invariants as unreachable!() #1890
• refactor(payload): mark fallible-context infallibles as unreachable!() #1885
• refactor(payload): mark infallible .expect() sites as unreachable!() #1884
• chore(clippy): drop expect_used quarantine for lading_throttle #1883
• chore(clippy): forbid .expect() in production code #1882
• main

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