chore(docs): document strict AgentExecutor streaming rules in v1.0 guide

[sokoliva]

Description

• Adds a new section "Server: AgentExecutor Streaming Rules" to the v0.3 → v1.0 migration guide, covering the breaking change introduced in feat!: Raise errors on invalid AgentExecutor behavior. #979 where the server now strictly enforces the A2A spec rules for message/stream.
• Documents the four InvalidAgentResponseError cases (mixing Message and Task events, multiple Messages, updates before the initial Task) with a clear table mapped to the actual error messages from active_task.py.
• Provides before/after migration examples — including the message-only pattern and the full task lifecycle pattern (Task → WORKING → invoke agent → artifact → COMPLETED) — plus a quick migration checklist.
• Links to the updated helloworld/agent_executor.py in a2a-samples#474 as a working reference.
• Adds a corresponding bullet to the "Summary of Key Changes" section

Why

Existing v0.3 executors that mixed Message and Task events were silently tolerated. After #979 they fail at runtime with InvalidAgentResponseError, which is easy to hit and hard to diagnose without guidance. This section gives users a clear path to migrate.

[gemini-code-assist]

Code Review

This pull request updates the v1.0 migration documentation to include new sections on AgentExecutor streaming rules and corrects various cross-references and typos. The documentation now explicitly details the strict enforcement of streaming patterns, requiring either a single message or a task-first lifecycle. One minor improvement was suggested to generalize a violation description in the streaming rules table to cover all task update events.