feat: add tracesSampler callback and integrations array to SentryConfig

[benminer]

Summary

Adds two new optional fields to SentryConfig to unblock the backend monorepo migration from direct Sentry/Pyroscope initialization to observability-js.

• tracesSampler — custom sampling callback invoked after the built-in ignoredRoutes check and before the default sampleRate fallback. Returns number | undefined; returning undefined falls through to sampleRate. Enables the backend to suppress traces for specific customers (e.g. QAWolf test accounts) without patching the library.
• integrations — array of additional Sentry integrations spread after the hardcoded defaults (expressIntegration, nodeProfilingIntegration). Enables consumers to register prismaIntegration(), rewriteFramesIntegration(), etc.

Both fields flow unchanged through resolveConfig into ResolvedConfig.sentry. Types are derived from the already-declared @sentry/node dependency via NodeOptions utility types — no new imports or dependencies added.

Also relaxes engines.node from >= 24 to >= 20 so the package installs cleanly on backends not yet on Node 24 (.nvmrc stays at 24 for local dev).

Testing

5 new tests in filtering.test.ts cover the sampler composition logic:

• fallback to sampleRate when no custom sampler is provided
• custom sampler return value overrides sampleRate
• undefined return falls through to sampleRate
• ignoredRoutes short-circuits before the custom sampler is called
• fractional sample rates pass through correctly