fix: replace z.undefined() with z.never() for JSON Schema compat

[notgitika]

Summary

Fixes Zod JSON Schema generation failure: z.undefined() cannot be represented in JSON Schema.

The AB test variant XOR union used z.undefined().optional() to exclude the alternate field. Replace with z.never().optional() which Zod can serialize.

Test plan

• npm run build passes
• node scripts/generate-schema.mjs succeeds

[agentcore-cli-automation]

LGTM. The change is minimal and correct:

• z.undefined() has no JSON Schema representation in Zod v4's z.toJSONSchema(), which is why generate-schema.mjs was failing.
• z.never().optional() serializes cleanly (as { "not": {} } on an optional property) and preserves the XOR semantics: the field must be absent/undefined, and any defined value is rejected on the inner schema. Combined with the union, an object with both fields set fails both branches, so mutual exclusivity still holds.
• No need to regenerate schemas/agentcore.schema.v1.json in this PR — the repo's release workflow regenerates it, and direct edits to schemas/ are rejected by CI outside release PRs.

[github-actions]

Coverage Report

Status	Category	Percentage	Covered / Total
🔵	Lines	42.89%	8936 / 20830
🔵	Statements	42.17%	9485 / 22491
🔵	Functions	39.66%	1537 / 3875
🔵	Branches	39.82%	5769 / 14486

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