fix(llm): size unknown models and refuse a section that cannot be served

Three defects surfaced while driving the Models page.

A hand-declared model needed an explicit contextWindow and maxTokens,
but a provider listing usually returns ids and nothing else — so the
page happily wrote a profile the adapter then rejected, which took the
whole namespace down silently. Capacities now fall back to the route's
`defaultContextWindow` (262,144) and `defaultMaxTokens` (32,768). Both
are guesses by construction, which is why they are route fields a
deployment corrects once rather than constants buried in the adapter;
the fallback sizes the model and never becomes a per-request cap.

That silent failure was the second defect. A schema-valid profile the
adapter could not serve was stored and only rejected later, disabling
every route in the namespace with nothing said. `dsh-settings` gains an
optional `validate` on registration — a check for what a schema cannot
express — and `llm-pi-ai` refuses an unserviceable section at the write
that produced it. A stored section that fails keeps the namespace's last
good value, as a schema failure already did, so an externally edited
document still cannot strand the owner. The plugin's own last-good
fallback goes with it: nothing reaching it can fail any more.

Third, a model with no reasoning metadata advertised the single level
`off`, which pi-ai translates to *omitting* the reasoning option — the
same request naming no effort produces. Selecting it disabled nothing,
so a provider whose default is to think kept thinking with `off` shown
as selected. Such a model now reports no reasoning capability at all,
which is the seam's way of saying the control is unavailable, and the
per-model `reasoning` flag is gone: without a thinkingLevelMap to spell
levels it could only invent them.

The protocol table narrows to the three a hand-declared route reaches
today, most-reached first so a surface offering a choice defaults to the
one gateways actually speak.

docs/config-catalog.md

@@ -718,6 +718,18 @@ export interface PiAiProviderProfile {
    * unset fields from the installed model of the same id.
    */
   models?: PiAiModelProfile[]
+  /**
+   * Context capacity for a model this route lists that neither the entry nor
+   * the installed catalog sizes (default 262,144). A guess by construction, so
+   * a deployment whose gateway serves smaller models corrects it here.
+   */
+  defaultContextWindow?: number
+  /**
+   * Output capability for a model this route lists that neither the entry nor
+   * the installed catalog sizes (default 32,768). This sizes the model; it
+   * never becomes a per-request cap on its own.
+   */
+  defaultMaxTokens?: number
   /** Provider request headers; Harness attribution wins reserved names. */
   headers?: Record<string, string>
   /** Provider-neutral pi-ai reasoning level. */
@@ -748,18 +760,17 @@ export interface PiAiModelProfile {
   contextWindow?: number
   /**
    * Maximum output tokens. Configuring one also makes it this model's
-   * per-request default; the value inherited from the installed catalog is the
-   * model's capability and never becomes a request default on its own.
+   * per-request default; a value inherited from the installed catalog, or the
+   * route's fallback, is the model's capability and never becomes a request
+   * default on its own.
    */
   maxTokens?: number
-  /** Whether the model exposes reasoning; defaults to the catalog capability. */
-  reasoning?: boolean
 }
 ```
 
 Depends on: `CacheRetention` (`@earendil-works/pi-ai`) · `ModelThinkingLevel` (`@earendil-works/pi-ai`) · [`RetryPolicyConfig`](../packages/llm/llm/src/index.ts) · `ThinkingBudgets` (`@earendil-works/pi-ai`) · `Transport` (`@earendil-works/pi-ai`)
 
-Source: [`packages/llm/llm-pi-ai/src/config.ts:104`](../packages/llm/llm-pi-ai/src/config.ts)
+Source: [`packages/llm/llm-pi-ai/src/config.ts:122`](../packages/llm/llm-pi-ai/src/config.ts)
 
 ## `@deepseek-ai/dsh-llm-replay`