chore(directus-client): reduce refresh coordinator internal API surface

[remihuigen]

Context

Follow-up cleanup from #280.

The refresh coordinator currently re-exports several timing constants and helpers from refresh-coordinator/index.ts, even though they are implementation details used only by the coordinator internals/tests.

This is not a correctness issue and does not need to block #280. We can handle it separately to keep the coordinator surface small and easier to reason about.

Proposed cleanup

• Stop re-exporting coordinator-internal timing values/helpers from refresh-coordinator/index.ts unless they are genuinely needed outside the coordinator implementation.
• Keep REFRESH_STORAGE_MOUNT internal if no external caller needs it.
• Rename the generic REFRESH_RESULT_TTL_SECONDS constant to something explicit such as REFRESH_TERMINAL_RESULT_TTL_SECONDS, since completed results now use a separate 30-second TTL and the 5-second value applies specifically to terminal failures.
• Keep the runtime behavior unchanged.

Current intended TTL semantics remain:

completed refresh: 30s
terminal failure:  5s
transient failure: 1s
Redis lease:       30s

Goal

Reduce unnecessary API-shaped surface and make the remaining names describe their actual semantics. Prefer the smallest possible diff; do not introduce new abstractions or redesign the coordinator.

Acceptance criteria

• Coordinator internals are not unnecessarily re-exported from index.ts.
• The 5-second terminal-result TTL has an explicit name.
• No refresh coordination behavior changes.
• Existing coordinator/auth tests continue to pass.

Related: #280