Desktop: widen Client Capability tool-schema allowlist so MCP tools using prefixItems/not/patternProperties/contentEncoding/if-then-else/deprecated are offered

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Summary

#4592 fixed two causes of "MCP tools fail to load on Desktop" (#4591): the native-capability offer now accepts jsonSchema() parameters, and each tool's descriptor is built and validated independently so one unrepresentable tool is skipped-and-warned instead of dropping every Desktop capability (Browser, Computer Use, Client settings, Rive, MCP) with it.

That isolation makes the outage survivable, but it does not make every MCP tool available: any tool whose JSON Schema uses a keyword outside the Client Capability allowlist is still skipped (offered to nobody). This issue tracks widening that allowlist so those tools are offered.

Where

packages/runtime-host/src/protocol/client-capability.ts — CLIENT_CAPABILITY_SCHEMA_KEYWORDS (the allowlist) and validateToolInputSchema (the per-keyword validation/recursion). A tool schema containing any key not in the set is rejected with Unsupported Client Capability tool schema keyword.

Missing keywords (from #4592 review)

Running real buildMcpTools() output through decodeClientCapabilityReplaceInput shows these ordinary keywords are absent and cause a skip:

• prefixItems — e.g. a pydantic tuple[...]
• contentEncoding (and typically contentMediaType) — e.g. pydantic bytes
• not, patternProperties, if / then / else
• deprecated

These are not exotic: packages/mcp/src/index.ts (normalizeToolInputSchema) already walks and preserves prefixItems, patternProperties, not, if/then/else, dependentSchemas, contains, additionalItems, unevaluatedItems/unevaluatedProperties, etc., so the repo already expects to receive them. The full gap between what the MCP normalizer preserves and what the protocol allowlist accepts is worth auditing in one pass.

Scope / considerations

• This is a protocol change. Editing packages/runtime-host/src/protocol/** requires bumping RUNTIME_HOST_COMPATIBILITY_EPOCH (the test CI job fails fast otherwise), so it needs deliberate protocol review — which is why fix(desktop): accept jsonSchema() tool params in native capability offers #4592 intentionally left it out.
• Each added keyword needs real validation, not just an allowlist entry: correct value type (prefixItems = non-empty array of schemas, contentEncoding = string, deprecated = boolean, not/if/then/else/patternProperties = schemas) and correct recursion in validateToolInputSchema, consistent with the existing items/allOf/properties handling.
• Keep the manifest size / tool-schema byte caps in mind.

Acceptance

• MCP tools whose schemas use the keywords above are offered on Desktop and survive decodeClientCapabilityReplaceInput.
• Protocol tests cover accept + reject for each new keyword.

Follow-up to #4592. Related: #4591.

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