feat(memory): support inline json prompt injection

[TylerBarnes]

Summary

Adds a new structuredOutput.jsonPromptInjection: 'inline' mode.

Before this PR, JSON prompt injection put the structured-output schema instructions in the system prompt (true / 'system'). That works, but it changes the system prompt for each schema and can defeat provider prompt caching.

This PR adds an inline mode that appends the JSON instructions to the latest user message instead, keeping the leading system prompt stable and more cache-friendly.

await agent.generate('Extract the task details', {
  structuredOutput: {
    schema: taskSchema,
    jsonPromptInjection: 'inline',
  },
});

Behavior by mode:

jsonPromptInjection: true      // inject into system prompt
jsonPromptInjection: 'system'  // inject into system prompt
jsonPromptInjection: 'inline'  // inject into latest user message

If there is no user message, inline mode adds a user message containing the JSON instructions. Native responseFormat is still skipped whenever prompt injection is enabled.

The structured-output fallback helpers preserve explicit 'inline' / 'system' settings when retrying, instead of coercing every retry to true.

Test plan

• Stack rebase verified linear against feat(core): add processor agent context #18651
• Covered by downstream memory/core checks in the stack

Stack

• Previous: feat(core): add processor agent context #18651
• Next: feat(memory): add observational memory extractors #18653

[changeset-bot]

🦋 Changeset detected

Latest commit: 0ede67b

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 22 packages

Name	Type
@mastra/core	Minor
mastracode	Patch
@mastra/mcp-docs-server	Patch
@internal/playground	Patch
@mastra/client-js	Patch
@mastra/opencode	Patch
@mastra/longmemeval	Patch
mastra	Patch
@mastra/deployer-cloud	Minor
@mastra/react	Patch
@mastra/playground-ui	Patch
@mastra/server	Minor
@mastra/deployer	Minor
create-mastra	Patch
@mastra/express	Patch
@mastra/fastify	Patch
@mastra/hono	Patch
@mastra/koa	Patch
@mastra/nestjs	Patch
@mastra/next	Patch
@mastra/tanstack-start	Patch
@mastra/temporal	Patch

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Project	Deployment	Actions	Updated (UTC)
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[dane-ai-mastra]

PR triage

Linked issue check skipped for core contributor @TylerBarnes.

---

PR complexity score

Factor	Value	Score impact
Files changed	9	+18
Lines changed	259	+15
Author merged PRs	564	-20
Test files changed	Yes	-10
Final score		3

Applied label: complexity: low

---

Changed test gate

Changed tests failed against the base branch as expected.

Label: tests: green ✅