test(traces): dedupe test preamble in trace_processor tests

[lucaspimentel]

Overview

Test-only refactor deferred from #1320 to keep that PR small.

Eight tests in traces/trace_processor.rs repeated the same setup: a Config, a Provider, a ServerlessTraceProcessor with an ObfuscationConfig, and a TracerHeaderTags value. This extracts that preamble into three shared test helpers inside mod tests:

• create_compute_stats_config(): the shared config with lambda_extension_compute_stats: true used by seven of the tests
• create_test_processor(&Arc<Config>, Arc<std::sync::Mutex<ErrorsSampler>>): builds the tags provider (lambda runtime, test-arn metadata) and the processor, taking the config and sampler as parameters
• create_test_header_tags(): the shared 'static tracer header tags

Accepting the config as a parameter lets test_process_traces_body_size_reflects_enriched_payload keep its deliberate default configuration (local stats disabled) while sharing the rest of the setup. Sampler choices are unchanged: six tests use the enabled error sampler, two use the disabled one. No production code changes.

Testing

• cargo test -p bottlecap traces::trace_processor::tests: same test count as before the refactor, all passing
• cargo test -p bottlecap: all unit and integration tests pass
• cargo fmt --all -- --check, cargo clippy --lib --tests, and git diff --check clean

[datadog-datadog-prod-us1-2]

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