fix(events): rename error event data key from message to text

[galuszkm]

Description

The ERROR event emitted by EventPublisher used data["message"] as the key for the exception string, while every other event that carries a human-readable string payload uses data["text"] (TOKEN, REASONING, AGENT_COMPLETE). Renames the key to text for consistency.

Related Issues

N/A

Type of Change

• Breaking change

YAML / API Impact

No schema or Python API changes. Breaking only for consumers reading event.data["message"] on ERROR events — they must switch to event.data["text"]. All other event shapes are unchanged.

Testing

• I ran uv run just check (lint + type check)
• I ran uv run just test for overall testing
• I added or updated tests that prove my fix is effective or my feature works
• I verified existing examples in examples/ still work

Checklist

• I have read the CONTRIBUTING document
• I have updated the documentation accordingly
• My changes generate no new warnings
• Any dependent changes have been merged and published

Note: Downstream consumers (e.g. strands-compose-agentcore) that read error.data["message"] need to be updated to error.data["text"].