feat: sampling, validator, and formatter spans via plugin hooks

[ajbozarth]

Background

Covers what retired sub-issues #471, #472, #473 were trying to do, rolled into one since the mechanism is identical across all three.

Scope

For each of sampling (BaseSamplingStrategy loop), validator (Requirement.check), and formatter (Formatter.apply / render_prompt / parse_response), add a plugin in tracing_plugins.py that hooks the existing pre/post hooks. The metrics plugins for these already exist (SamplingMetricsPlugin, RequirementMetricsPlugin) — mirror their structure.

• Use semconv-compliant attribute names where defined; fall back to mellea.* for custom.
• Each plugin can land as its own PR if preferred (this issue collapses the three only for planning purposes; the implementation can be split if it's cleaner).

Phase & dependencies

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

Acceptance criteria

• Sampling iteration, requirement check, and formatter pass each produce spans with appropriate parent-child relationships.
• Tests cover span emission for each component.

Parent epic: #444