feat: add AbortSignal support to Agent and BedrockModel

[jsamuel1]

Summary

Plumb AbortSignal through the invocation stack so callers can cancel in-flight requests:

• InvokeOptions.signal passed to Agent.invokeModel
• StreamOptions.signal passed to Model.stream
• BedrockModel forwards signal as abortSignal to AWS SDK client

Usage

const controller = new AbortController()
const result = await agent.invoke('Hello', { signal: controller.signal })

// Cancel from another context:
controller.abort()

Follow-up needed after multi-agent PRs merge

Once the swarm (#587) and graph (#594) multi-agent PRs land, abort signal propagation should be added to both:

• Swarm: Currently accepts signal?: AbortSignal in SwarmInvokeOptions and checks signal.aborted at the top of the loop, but does NOT forward the signal to node.agent.stream(input). It should pass { signal: options.signal } to each agent invocation so in-flight model calls are actually cancelled.
• Graph: Has no abort signal support at all. Should accept a signal option, check it between node executions, and forward it to each agent invocation.

[jsamuel1]

Note: AbortSignal threading through Graph (passing signal down to node.stream()) depends on this PR's changes to Node.stream() accepting a signal parameter. The Graph PR (#594) already adds signal? to the public invoke()/stream() signatures per MultiAgentBase, but defers the internal threading until this PR merges.