feat(telemetry): name agent invocation spans

[jcarver989]

Summary

• name agent invocation spans with the configured agent name
• record gen_ai.agent.name while keeping gen_ai.operation.name set to invoke_agent
• preserve invoke_agent as the fallback when no agent name is available

Review focus

Agent identity stays separate from the W3C trace propagation context. ObserverFactory::agent receives the name and parent context as explicit arguments.

Walkthrough

• 277c597a: thread agent names into telemetry and cover named parent and child agent spans

Verification

• just test
• cargo check --workspace
• cargo clippy -p aether-agent-core -p aether-telemetry --all-targets --all-features --no-deps -- -D warnings
• cargo fmt --check --all