feat: upgrade A2A templates to a2a-sdk 1.0 protocol spec

[aidandaly24]

Description

The a2a-python SDK has released v1.0 which implements the A2A 1.0 protocol spec. Our A2A templates (GoogleADK, Strands, LangChain+LangGraph) currently use 0.3-era APIs and are pinned to < 1.0.0 (see #1145).

We need to migrate all three A2A templates to the 1.0 SDK, which includes:

• Updated module paths (e.g., a2a.server.agent_execution, a2a.server.events, a2a.server.tasks likely restructured)
• Renamed/restructured types (AgentCapabilities, AgentCard, AgentSkill, Part, TextPart, etc.)
• Updated a2a.utils usage (e.g., new_task)
• Compatibility with bedrock-agentcore[a2a] runtime integration (serve_a2a())

Acceptance Criteria

• All three A2A templates (googleadk, strands, langchain_langgraph) updated to use a2a-sdk >= 1.0.0 APIs
• pyproject.toml version constraints updated to a2a-sdk >= 1.0.0 (remove < 1.0.0 pin)
• agentcore create --protocol A2A produces working projects for all three frameworks
• agentcore dev successfully starts the A2A agent server for each template
• bedrock-agentcore[a2a] runtime (serve_a2a()) remains compatible or is updated accordingly
• Snapshot tests updated
• Integration tests pass

Additional Context

• PyPI versions: https://pypi.org/project/a2a-sdk/#history (1.0.0, 1.0.1, 1.0.2 available)
• Migration may also require coordinated changes in bedrock-agentcore-sdk-python if serve_a2a() depends on 0.3 SDK internals
• PR fix: pin a2a-sdk below 1.0 to prevent breaking changes #1145 is the short-term fix pinning to < 1.0.0