Propagate ThreadLocal context to streaming tools

[luyu0425]

Summary

Preserve registered ThreadLocal context across the boundedElastic thread
hop used for streaming tool execution.

Streaming tool calls execute blocking tool callbacks on a boundedElastic
worker. As a result, thread-local state from the subscribing thread, such as
servlet request attributes, is otherwise unavailable during tool execution.

The change captures registered Micrometer ThreadLocalAccessor state at
subscription time, carries it through Reactor Context, and restores it around
tool execution.

It also:

• clears missing thread-local state to prevent stale worker context from leaking between requests
• excludes Observation state so the existing Spring AI tracing hierarchy is preserved
• registers RequestAttributesThreadLocalAccessor only for servlet web applications
• manages that accessor for the lifetime of the application context

Tests cover subscription-time capture, recursive tool-call rounds, scope
cleanup, stale worker state, observation exclusion, and servlet
auto-configuration lifecycle.

Fixes #3877

Testing

• ./mvnw -pl spring-ai-client-chat -am test
• ./mvnw -pl auto-configurations/models/chat/client/spring-ai-autoconfigure-model-chat-client -am test
• ./mvnw clean package