[dop] [error-tracking]: derive debug_id from file content

[jhssilva]

Summary

The RUM plugin injects a debug_id into every built chunk's own JS content. This PR makes the error-tracking plugin extract that debug_id back out of each file when it uploads that file's sourcemap, instead of relying on the previous filename-based coordination between the two plugins.

Why

Previously, error-tracking got the debug_id from a side-channel Map that the RUM plugin populated at injection time, keyed by the chunk's filename. Bundlers (webpack/rspack) rename chunks after injection via realContentHash, so by the time error-tracking looked the value up by the chunk's final filename, the key no longer matched — the debug_id was silently dropped for almost every chunk in a real build.

How

Instead of coordinating through a filename-keyed Map, error-tracking now reads each minified file's own content directly and extracts the debug_id from the ddDebugId literal the RUM plugin already embeds inside it. This works no matter what renaming the bundler does afterward, since the debug_id travels with the file's content, not its name.

• error-tracking/src/sourcemaps/debugId.ts (new): extractDebugId() — regex-extracts the debug_id from a file's content.
• error-tracking/src/sourcemaps/sender.ts: reads the minified file's content and extracts the debug_id from it, in place of the old Map lookup.
• plugins/rum/src/index.ts + getSourceCodeContextSnippet.ts: RUM no longer needs to track debug_ids separately — the value already lives in the code it injects.
• core/src/helpers/fs.ts: new readFilePrefix() helper, since we only need the first bytes of each file, not the whole thing.
• plugins/injection/{xpack,esbuild}.ts: keep injected chunk filenames as relative paths for consistency with the rest of the pipeline.

Notes

• Real end-to-end build against a module-federation-remote entry (mf_apm_runtime_app, rspack), uploaded to staging: each nested chunk's uploaded debug_id matches the ddDebugId literal embedded in that chunk's own file.

[datadog-datadog-prod-us1-2]

🎉 All green!

🧪 All tests passed
❄️ No new flaky tests detected

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[rgaignault]

Suggestion: With esbuild, this runs before the injection onEnd, so it uploads the pre-injection files without ddDebugId. Should't we defer the upload until injection completes.

[jhssilva]

For now, we'll be dogfooding in web-ui. We'll do other iterations as needed in future pr's.
We'll keep this in mind.