feat: tool call spans via tool_post_invoke hook

[ajbozarth]

Background

Tool calls have no tracing today. The tool_post_invoke hook exists (consumed by ToolMetricsPlugin) and is the right attach point. Replaces retired sub-issue #474.

Scope

• Add a ToolTracingPlugin in tracing_plugins.py that emits a tool.call span on tool_post_invoke (and a companion pre-invoke hook if one exists, or a small prerequisite issue to add one).
• Attributes: gen_ai.tool.name, gen_ai.tool.call.id (semconv), plus Mellea-specific mellea.tool.arguments_hash, mellea.tool.status.
• Make tool spans children of the backend generation span when possible.

Phase & dependencies

Phase 2 (coverage). Depends on Phase 1 (#1045, #1046, #1047). Can proceed in parallel with {{NEW-D}}, {{NEW-F}}, {{NEW-G}}, {{NEW-H}}, and with PR #1036 rebased onto Phase 1.

Acceptance criteria

• Every tool invocation emits a span.
• Span attributes follow OTel GenAI semconv where defined.

Parent epic: #444