Splice observed shapes into the MCP result envelope

[RhysSullivan]

Follow-up to #1759, covering the case the Blacksmith post actually benchmarked: MCP tools whose servers declare no output schema. Those tools were excluded from muscle memory because the MCP plugin always synthesizes a CallToolResult envelope schema — declared as far as the serve rule could tell, but with an untyped structuredContent placeholder.

Slot mechanism (generic, in the serving layer) — SHAPE_SLOT_KEY (x-executor-shape-slot) marks a placeholder subschema inside a declared schema. tools.schema now has a second serve path: when a declared output schema carries marked slots, the observed shape's counterpart at the same path is spliced into exactly those slots, keeping the declared structure around them. Markers are stripped before serving either way; outputSchemaSource: "observed" is set only when a slot was actually filled. Kept out of plugin internals deliberately so it ports through the planned plugin-system removal.

MCP plugin — implements the previously-unused projectToolSchema hook: when the persisted envelope's structuredContent is the synthesized placeholder (identified by required not listing it, which only happens when the server declared nothing), the slot is marked at read time. No persisted-row change, so every existing catalog row gets the behavior immediately, no refresh needed.

Evidence — new integration test (real MCP server over streamable-http through a real executor): cold describe shows the envelope with an untyped slot and no marker leakage; one call; warm describe serves structuredContent: { value: string; length: number; ok: boolean } with outputSchemaSource: observed. New e2e scenario (selfhost + cloud, both passing) drives the same journey through the public surfaces with the provenance note asserted. 6 new splice unit tests. New makeUndeclaredStructuredMcpServer testing fixture.

Known limitation — servers that return their payload as JSON text inside a content block (rather than structuredContent) still teach only text: string; shape inference does not parse string contents. Typing those would require bounded JSON-in-string inference (contentSchema) — a possible follow-up, noted here so the warm-describe win isn't overstated for text-only servers.

Local note: full unit suite green per package; the fully-parallel local run hit 5s-timeout/port-cross-talk flakes in unrelated OAuth-flow tests (pass in isolation) — CI shards are the gate.

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commit: 4566b5a

[RhysSullivan]

Superseded by the MCP data-contract change (semantic data + core-owned result encoding): with data unwrapped to the payload, MCP tools fall under the plain undefined-schema serve rule and the slot splice is unnecessary. Test fixture and e2e journey carry over to the new PR.