Add detailed tracing to session operations beyond the current top-level context span

[psschwei]

Description:
Add detailed tracing to session operations beyond the current top-level context span.

Detailed Requirements:

1. Add spans for key session methods:
   • session.act - Action execution
   • session.instruct - Instruction-based generation
   • session.chat - Chat interactions
   • session.validate - Requirement validation
   • session.sample - Sampling orchestration
2. Add attributes:
   • action_type - Type of action being executed
   • instruction - Truncated instruction text (if privacy allows)
   • validator_count - Number of validators
   • strategy - Sampling strategy name
3. Maintain parent-child span relationships
4. Use existing trace_application helper

Files to Modify:

• mellea/stdlib/session.py - Add spans to methods
• mellea/core/mfuncs.py - Instrument core functions if applicable

Span Hierarchy Example:

session_context (existing)
├── session.instruct
│   ├── backend.request
│   └── formatter.apply
├── session.validate
│   └── validator.check (per validator)
└── session.sample
    ├── sampler.iterate
    └── ...

Acceptance Criteria:

• All session methods have corresponding spans
• Spans correctly nested under session context
• Relevant attributes captured
• No performance impact when tracing disabled

[ajbozarth]

Closing as part of the tracing epic (#444) re-plan. This sub-issue predates the plugin/hooks pattern (PR #653) and the GenerationMetadata partition (PR #908); its spec has drifted.

Replacement: #1048 — session and act spans via plugin hooks (removes the direct tracing imports in stdlib/session.py / stdlib/functional.py).

See #444 for the full refreshed plan.