feat: add rebased telemetry implementation

[lizradway]

Description

• Add approved telemetry implementation from /liz/otel branch rebased with structured output changes
• The only rebasing with real ramifications was done under agent.ts

Differences between /liz/otel branch :

Structural: _executeAgentLoop inlined into _stream

• The original branch extracted the agent loop into a separate _executeAgentLoop method, with _stream handling the agent span lifecycle around it.
• After rebase, structured output's context.registerTool() / context.cleanup() must wrap the loop, and the agent span must wrap both.
• Keeping them as separate methods would require threading the structured output context through or modifying scoping.
• Inlining the loop into _stream keeps a single try/finally that handles both telemetry span cleanup and structured output cleanup.
• Structured output logic and telemetry logic should be extracted out in a follow up as we consider modularization of the agent loop similarly to Python

Similarly:

• The original branch returned result from _executeAgentLoop and passed it to endAgentSpan in _stream's finally.
• With the loop inlined, result is declared as let result: AgentResult | undefined at the top of _stream so the finally block can reference it for the agent span's response/stopReason attributes.

Force tool continue

• Structured output introduces a continue path (when forcing the model to use the structured output tool see line 473).
• The inner try/catch around each cycle only catches errors, so continue would skip endAgentLoopSpan. Added an explicit endAgentLoopSpan(cycleSpan) before the continue to prevent span leaks.
• Added test coverage for this interaction and other structured output interactions

_appendMessage is synchronous/ returns event for yielding

• The original /liz/otel branch had async _appendMessage that invoked hooks directly.
• Main's event wrapper PR changed this to a synchronous method that returns a MessageAddedEvent for yielding — hooks are invoked centrally in stream(). The rebase uses the current synchronous pattern.

Related Issues

#69

Documentation PR

strands-agents/docs#557

Type of Change

New feature

Testing

How have you tested the change?

• I ran npm run check

Checklist

• I have read the CONTRIBUTING document
• I have added any necessary tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly
• I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• My changes generate no new warnings
• Any dependent changes have been merged and published

---

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[poshinchen]

We can remove this and use setAttributes from the span API.