refactor(config): rename compute_trace_stats_on_extension -> lambda_extension_compute_stats

[duncanista]

Summary

Renames the public env var `DD_COMPUTE_TRACE_STATS_ON_EXTENSION` to `DD_LAMBDA_EXTENSION_COMPUTE_STATS`, plus the matching Rust field, so the name reflects the actual scope: this toggle only governs the Lambda extension's APM trace-stats pipeline and is not a generic Datadog Agent concept.

Why

Auditing the `LambdaConfig` extension fields surfaced this one as the one with an ambiguous env var name. "Compute trace stats on extension" reads as if there's a generic "extension" concept; in practice it's specifically the Lambda extension. Making that explicit avoids confusion if/when the same idea surfaces in other embedders (which would then need a differently-named knob).

Changes

• `LambdaConfigSource` field renamed: `compute_trace_stats_on_extension` → `lambda_extension_compute_stats`. Figment's `DD_` prefix maps to the new env var automatically.
• `LambdaConfig` field renamed in lockstep. All call sites in `trace_processor.rs`, `otlp/agent.rs`, `lifecycle/invocation/processor.rs`, `tags/lambda/tags.rs`, and the integration test now read `config.ext.lambda_extension_compute_stats`.
• `merge_from` keeps the field in the existing `value:` group of `merge_fields!` — no source-to-config rename needed since field names match.
• No back-compat alias for the old env var. The `.ext` migration this depends on has not shipped, so the legacy name was never released externally.
• Test renamed; default-false test added.

Test plan

• `cargo test` — `config::lambda_config_tests::lambda_extension_compute_stats_from_env` and `lambda_extension_compute_stats_defaults_false` pass; 38/38 in the module.
• `cargo clippy --workspace --all-targets --features default` — clean.
• `cargo fmt -- --check` — clean.

Notes for review

Stacked on top of `feat/migrate-bottlecap-to-upstream-config` (the .ext migration PR). Merges into that branch.

[lucaspimentel]

LGTM aside from the question about keeping DD_COMPUTE_TRACE_STATS_ON_EXTENSION as a fallback for backwards compatibility.