Support caller-provided tool declarations in direct agent requests

[astrobot-houston]

Originally proposed by @MikeRyanDev in #185. Their description is reproduced verbatim below.

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Summary

• add DirectAgentPayload.tools and exported DirectAgentToolDeclaration types to runtime and SDK surfaces
• validate direct HTTP tool declarations, include them in the OpenAPI request schema, and plumb them into direct-agent prompt processing
• expose these tools to the model as declaration-only prompt tools; execution currently throws a clear unsupported error

Follow-up

• browser/client deferred tool execution and resume semantics are still future work
• this PR only carries caller-scoped tool declarations through the direct-agent request path

Verification

• corepack pnpm --dir packages/runtime run check:types
• corepack pnpm --dir packages/runtime exec vitest run test/direct-agent.test.ts
• corepack pnpm --dir packages/sdk run check:types
• corepack pnpm --dir packages/sdk exec vitest run test/client.test.ts

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Original implementation from #185 by @MikeRyanDev

[MikeRyanDev]

Hi folks! 👋 I work on AG-UI at CopilotKit. I've been experimenting with an AG-UI adapter for Flue agents. One area of incompatibility is frontend tools, where a given frontend can add UI-specific tool declarations to an agent. The linked PR was an exploration of what adding these tools via the SDK would look like.

Let me know what your preferred design process is. I'd love to carry this work forward.