chore(SDK-971): fix release tags and delete unused code

[mariechatfield]

Summary

Jira: SDK-971

Cleanup pass on the phase 1 TSDoc backfill (#1996), correcting release tags that were set incorrectly and removing dead code surfaced by running knip.

Release tag corrections (@public → @internal)

The following are internal SDK plumbing that partners never interact with directly — they were incorrectly tagged @public in the backfill:

• LocaleProvider, LocaleProviderProps, LocaleProps, useLocale — consumed by GustoProvider, not partners
• ObservabilityProvider, ObservabilityProviderProps, ObservabilityContextValue, useObservability — same pattern

Also removes LocaleProps and ObservabilityProviderProps / ObservabilityContextValue from their index re-exports, and unexports CtaConfig (was @internal but still exported) and removes normalizeToSDKError from the ObservabilityProvider barrel (wrong module for it).

BaseLayoutProps and BaseBoundariesProps removed from the Base barrel — internal component shape types not part of the partner API.

Dead code removal

Hooks surfaced as unused by knip:

• useDebounce — deleted (no remaining callers)
• useForkRef — index deleted (implementation file stays; it's used but the barrel re-export was unused)
• useOverflowDetection — barrel index deleted, default export removed from implementation
• BreakpointKey type — removed from useContainerBreakpoints (unused export)

API report

.reports/embedded-react-sdk.api.md updated to reflect the above — the newly-internal symbols now appear with @internal and the ae-internal-missing-underscore warning (expected; we don't rename internal symbols with a leading underscore).

Test plan

• npm run build passes
• npm run test -- --run passes

[mariechatfield]

These are the actual code changes that inspired the skill and agent changes in #2009