Privacy: Add @since tags and clarify JSDoc in privacy-tools.js

[zakariabinsaifullah]

Follow-up to r63556, which added docblocks for the privacy request helper functions in privacy-tools.js. This PR covers the rest of the review feedback left on #6690 (by @masteradhoc, @VladimirAus and @mukeshpanchal27):

• @since tags on every function. I checked versions against the release tags. The helpers were introduced in 4.9.6 (in wp-admin/js/xfn.js at first, moved to privacy-tools.js in 5.3). setExportProgress() and setErasureProgress() were added in 5.4.0. The signatures haven't changed since.
• Optional params: zipUrl and errorMessage are now marked [optional], and the descriptions say what happens when each is omitted.
• Index descriptions: the progress functions receive the number of completed exporters/erasers (0 to exportersCount/erasersCount). doNextExport()/doNextErasure() receive a 1-based index (the first call is doNextExport( 1, 1 )). The summaries now say what drives the progress update.
• onErasureFailure() was missing its @return {void}.

Documentation only; no behavior change.

Checks run: npx eslint, npx wp-scripts lint-js (JSDoc rules), grunt jshint:core, node --check, git diff --check. All pass.

Trac ticket: https://core.trac.wordpress.org/ticket/49381

Use of AI Tools

AI assistance: Yes
Tool(s): Claude Code
Model(s): Claude Opus 5.5
Used for: Researching @since versions from the release history and drafting the docblock wording. I reviewed the final changes and take responsibility for them.

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[masteradhoc]

Thanks! This looks good to me.

[mukeshpanchal27]

Look solid to me.