APMSVLS-494: add a standalone fake intake for local APM debugging

[lucaspimentel]

Overview

Promotes the test-only fake-intake fixture into the bottlecap library behind a new fake-intake feature and adds a standalone fake-intake binary for local APM debugging (APMSVLS-494 phase 2).

The new binary accepts the same APM endpoints the extension flushes to (/api/v0.2/stats msgpack, /api/v0.2/traces protobuf, /api/v0.1/pipeline_stats DSM msgpack), decodes payloads on arrival, and reports them locally. It supports:

• Request summaries: one stderr line per handled request with identity, endpoint, encoding, status, and payload count; stats requests additionally group hits by the full aggregation key (client env/version/origin context plus all grouped dimensions, deterministic ordering).
• Failure injection: FAKE_INTAKE_FAIL_STATS_FIRST_N returns HTTP 500 for the first N stats attempts (still decoded, summarized, and dumped, but not captured) so rejection/retry behavior can be exercised locally.
• JSON dumps: FAKE_INTAKE_DUMP_DIR writes one collision-proof envelope per decoded request, including rejected stats attempts.

The fake-intake feature is never enabled in default or fips. The APM and DSM integration tests now use the library module and are gated on the feature via required-features, so default/FIPS test runs skip them cleanly. Existing test bodies are unchanged. fake_intake.rs is self-contained for later extraction into apm-agent-parity-rs.

Local workflow (feature-gated builds, env vars, smoke procedure against bottlecap-test-mode) is documented in bottlecap/README.md.

Testing

• cargo nextest run --workspace --features fake-intake: 708/708 passed (includes the APM/DSM integration tests and new fake-intake unit tests for grouping, failure injection, concurrency, dumps, and config errors); default-feature run: 670 passed with the gated targets skipped.
• cargo clippy --workspace --all-targets -- -D warnings passes for default, default,test-mode,fake-intake, and fips.
• Manual smoke against the bottlecap-test-mode binary: trace POST accepted, /flush 204, stats hits 2/1 for GET /a/GET /b verified in summaries and JSON dumps; with FAKE_INTAKE_FAIL_STATS_FIRST_N=1 the stats request returned 500 then 202 on retry, both attempts dumped.
• GitLab pipeline template rendered with gomplate; new cargo test job and extended clippy line verified.

[datadog-prod-us1-4]

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[lucaspimentel]

Superseded by #1392, which consumes the intake as the shared datadog-mock-intake crate from DataDog/serverless-components#174 instead of a bottlecap library module. The standalone fake-intake binary, failure injection, summaries, and JSON dumps carry over unchanged; the APM/DSM integration tests now run in every feature set via the shared crate.