arch(llm): llm_dispatch.rs filter_map silently discards unknown ThinkingBlock variants

[bug-ops]

Description

In crates/zeph-core/src/agent/tool_execution/llm_dispatch.rs, unknown ThinkingBlock variants
introduced after #[non_exhaustive] was added to ThinkingBlock are discarded via filter_map
with _ => None.

This is the only forward-compatible option since no MessagePart variant exists to hold an
unknown ThinkingBlock shape. However, if a new ThinkingBlock variant carries data that should
influence tool dispatch (e.g. reasoning steps that affect tool selection), it will be silently
dropped with no log.

Suggested fix

Add a tracing::debug! log in the _ => None arm so that discarded unknown variants are
observable in traces without blocking compilation.

Environment

• File: crates/zeph-core/src/agent/tool_execution/llm_dispatch.rs
• Introduced by: arch(llm): StreamChunk, ChatResponse, ThinkingBlock, MessagePart missing #[non_exhaustive] #4515 / PR refactor(llm,commands,plugins,sanitizer,skills): add #[non_exhaustive] to extensible pub enums #4533