chore(v7): remove dead exports and unreachable telemetry middleware

[tusharpandey13]

Description

Cleanup follow-up to the v7 auth-separation work (#1390), which orphaned several public exports and a middleware hook. Tracked in the deferred-items comment on #1390.
See #1390 (comment)

Remove dead exports (breaking, must land within v7):

• ResponseError and FetchError (src/lib/errors.ts) and JSONApiResponse (src/lib/models.ts) were exported from the package entrypoint but no longer back any live code path after the auth layer was removed. TimeoutError is kept: it is still used by the retry path (src/lib/retry.ts).

Remove unreachable telemetry middleware:

• The pre() hook on Auth0ClientTelemetry is dead after the middleware pipeline was dismantled. Removed it along with the now-unused Middleware/FetchParams/RequestContext imports and the implements Middleware clause. Only getAuth0ClientHeader() is still called (from auth-helpers.ts). The class is internal-only (never re-exported), so this is not a public-contract change.

Document and test the fetch option:

• fetch is already a typed public option on ManagementClient (inherited from the base client, not excluded by the options Omit). Added a test asserting Management API requests actually route through a user-supplied fetch, and documented the option in the README.

Notes / out of scope

• Middleware/FetchParams/RequestContext remain public (they back Configuration.middleware). Deciding whether to remove that surface too is a separate call.
• Token-path domain validation and the migration-guide removed-exports list are tracked separately (the latter is owed in [BREAKING] feat!: remove Authentication layer #1390 itself).
• CHANGELOG entry for the breaking removals to be added before the GA cut.

Testing

• tsc --noEmit clean.
• Management wrapper unit suites: 559/559 passing, including the new management-client-fetch-option test.

🤖 Generated with Claude Code

[tusharpandey13]

Reviewer notes (beyond the description)

Points that are not obvious from the diff:

• TimeoutError is intentionally kept exported. It is still thrown by the retry path (src/lib/retry.ts:93), so it is not dead like the others. An earlier plan draft floated hiding it from the entrypoint; dropped on purpose to avoid converting the export * barrel to named re-exports. No doc claims it was removed, so no stale guidance.
• Telemetry edit is regen-safe. src/lib/ is under .fernignore, so the hand edit to auth0-client-telemetry.ts will not be clobbered on the next Fern generation. Auth0ClientTelemetry is never re-exported, so removing pre() is not a contract change.
• fetch was already public before this PR (inherited from the base client, not excluded by the options Omit) — the new test and docs cover existing surface, they do not add it.
• Removable-but-left-alone: VoidApiResponse (models.ts) and the dead src/lib/index.ts barrel also look orphaned; left out to keep the diff scoped. Flagging so they are not mistaken for oversights.
• CI noise: the repo-wide MSW/wire suites show pre-existing load errors (msw import), unrelated to this change — 0 actual test failures. Management wrapper unit suites are green (559/559).