feat: cursor-based pagination DSL, bounded result shape enforcement, and provider concurrency

[raphael]

Summary

This PR introduces three related improvements to goa-ai:

1. Cursor-Based Pagination DSL

New DSL functions allow tools to declare cursor-based pagination fields:

Tool("search", "Search docs", func() {
    Args(SearchArgs)
    Return(SearchResult)
    BoundedResult(func() {
        Cursor("cursor")        // payload field
        NextCursor("next_cursor") // result field
    })
})

Contract:

• Cursors are opaque; callers must not parse or synthesize them
• When paging, keep all other arguments unchanged; only set the cursor field
• Both fields must be optional Strings in their respective schemas

2. Bounded Result Shape Enforcement

BoundedResult() now enforces canonical bounds fields on tool results:

Field	Type	Required
returned	Int	✓
truncated	Boolean	✓
total	Int	optional
refinement_hint	String	optional

Authoring rule: Either declare none of these fields (let BoundedResult() add them) or declare all of them. Hybrid declarations are rejected at DSL eval time.

3. Provider Concurrency & Health Responsiveness

The toolregistry/provider.Serve function now uses a worker pool architecture:

• Non-blocking subscription loop: drains the toolset stream without blocking on tool execution
• Health ping responsiveness: pings are handled promptly even when all workers are busy
• Configurable concurrency: MaxConcurrentToolCalls and MaxQueuedToolCalls options
• Ack grace period: SinkAckGracePeriod for deployments with long-running tools

Additional Changes

• Reminder injection fix: Respects Bedrock constraint that tool_use must be immediately followed by tool_result (no injecting system reminders between them)
• Agent child result collection: Uses Await + IsReady to process child completions as they arrive rather than blocking sequentially
• Tool result reminders: New templates for retry hints and bounds reminders (injected backstage to guide the planner)

Testing

• Added provider tests verifying ping responsiveness while tools are in-flight
• Added runtime tests for agent child ordering and mixed batch execution
• All existing tests pass

Documentation

• Updated docs/dsl.md with pagination DSL and bounds field contracts
• Updated docs/overview.md and docs/runtime.md with bounded result details