Studio: surface external-provider cache hits and writes in context bar (#5736)

* Studio: surface external-provider cache hits and writes in context bar

The Anthropic / OpenAI Responses streaming paths already emit an
include_usage-style SSE chunk carrying prompt_tokens_details.cached_tokens
and cache_creation_input_tokens / cache_read_input_tokens (see
_build_usage_chunk in external_provider.py), but the chat-adapter only
read the local llama-server timings.cache_n field. As a result, the
context-usage tooltip never showed cache hits or writes for external
providers, even though the backend was computing them.

Read the external usage envelope as a fallback when timings.cache_n is
absent, and surface Anthropic cache_creation_input_tokens as a separate
"Cache writes" line in the tooltip so users can tell a cache miss from a
cache hit on a turn that both reads and writes the cache.

- ServerUsage gains optional prompt_tokens_details.cached_tokens,
  cache_creation_input_tokens, cache_read_input_tokens.
- contextUsage store entry gains optional cacheWriteTokens.
- ContextUsageBar gains optional cacheWrites tooltip line.
- chat-page wires both fields through to the bar.

* Studio: render cache stats for external providers too

Reviewer round on the original PR caught three asymmetric-fix sites
where the producer side surfaced external prompt-cache stats but the
consumer side still gated on ggufContextLength (which is only ever set
for the local llama-server runtime). Result: the entire cache-stats
PR shipped invisible for Anthropic / OpenAI Responses / Gemini, which
is exactly the set of providers it was added for.

- chat-page.tsx: drop the ggufContextLength precondition on the
  ContextUsageBar mount. The bar already tracks usage; let it decide
  what to render based on what it knows.
- context-usage-bar.tsx: make `total` optional. When absent, drop the
  "/ total" ratio + percentage progress bar + "approaching limit"
  helper, and just show per-turn counters + cache stats. Bootstrap
  guard tightened so an all-zero, all-undefined state still renders
  nothing.
- runtime-provider.tsx: external-provider rehydration was rejected by
  the `store.ggufContextLength` check. Keep the "fits inside window"
  sanity check when a local context window IS known, drop it when
  it isn't.
- message-timing.tsx: the per-message timing popover used a separate
  "Cache hits" code path that only read llama-server's timings.cache_n.
  Fall through to custom.contextUsage for external providers, and add
  a parallel "Cache writes" line for Anthropic cache_creation events.

* Studio: tighten cache-stats comments

* Scope contextUsage to active checkpoint

Three follow-ups on #5736 so the relaxed external-provider render
gate does not show stale token / cache stats from a different model:

1) setCheckpoint now clears contextUsage on a real checkpoint
   change. setActiveThreadId and clearCheckpoint already did this;
   the most-traveled transition path (the user switching models from
   the picker) leaked the prior turn's counts because they were never
   cleared.

2) The external-selection branch in chat-page.tsx now also clears
   contextUsage at the same time it nulls ggufContextLength /
   activeNativePathToken. Without this an in-session switch from a
   local model to an external provider would visibly carry the
   previous local turn's counters into the new provider's bar.

3) exitCompare's rehydration is now scoped: restore the saved
   usage only when the message's modelId matches the active
   checkpoint AND, for local turns where a context window is known,
   when the saved total fits inside that window. Without this the
   bar could render a stale local-model usage on top of an external
   provider, or an oversized usage object that exceeds the now-
   active window.

Typecheck clean.

* Plug remaining stale-contextUsage paths

Follow-up to 042e0ac4 that catches four asymmetric-fix sites the
checkpoint-scoping pass missed:

1) setParams now also clears contextUsage on a real checkpoint
   change. The local model load path in use-chat-model-runtime calls
   setParams(mergeBackendRecommendedInference(...)) which mutates
   params.checkpoint before refresh() eventually fires setCheckpoint;
   the intermediate window rendered the previous model's counters
   under the new checkpoint.

2) chat-adapter.ts setContextUsage on stream completion now gates on
   the captured params.checkpoint still being active. A late
   completion from provider A used to clobber the context bar after
   the user switched to provider B mid-stream.

3) chat-page.tsx exitCompare rehydration no longer accepts a saved
   modelId-stamped usage when the active checkpoint is empty. A user
   who entered compare, cleared the model, and exited compare would
   otherwise see the cleared model's stats reappear.

4) runtime-provider.tsx thread-load no longer restores legacy
   unscoped usage (no modelId) unless a local context window is
   known. With the relaxed external-provider render gate, old
   pre-PR persisted messages without a modelId stamp could attach
   their counts to an unrelated active provider.

Also switches message-timing.tsx cache-hit fallback from || to ??
so an explicit cache_n=0 is not replaced by a stale cachedTokens.

Typecheck clean.

* Shorten cache-stats comments for PR #5736