MCP presenter: ambiguous legacy origin returns a detailed error instead of the fixed body-free processing error #4248

S4 — Legacy error envelope differs from the approved exact rule

packages/coding-agent/src/core/extensions/builtin/mcp/elicitation.ts:76-79,98-106 gives the required body-free -32603 / Elicitation processing unavailable for no presenter. But ambiguous legacy origin takes a separate branch at :88-95 with explanatory text and data.reason:"ambiguous_legacy_origin". The approved presenter design (security review round 3, held locally) requires the same fixed no-data envelope for ambiguity too.

Small engine reconciliation PR: normalize only that enforced failure branch and its owner tests (packages/coding-agent/src/core/extensions/builtin/mcp/elicitation.ts, packages/coding-agent/test/mcp/presenter-rpc.test.ts). RED: ambiguous enforced legacy request emits fixed processing error without data. This is a design discrepancy, not evidence that prompt/schema content currently leaks there. If an explicit superseding approval exists, attach it and update the design instead; this investigation did not establish one.

Related: #4197, #4210 (merged), sisyphuslabs/omo-desktop-app#4370. Found while planning the desktop presenter integration.