[codex] add Claude/Amp Laminar usage traces

[Zygimantass]

Summary

Add Laminar-priced usage spans for Claude and Amp block-mode harness turns when their native JSON streams expose real token usage.

Changes

• Parse normalized token usage from Anthropic-style assistant/result events, including Claude fields such as input_tokens, cache_creation_input_tokens, cache_read_input_tokens, and output_tokens.
• Thread block-mode trace context through the generic Claude/Amp harness server path.
• Export one harness-owned OTLP *.session_task.turn span at turn completion with gen_ai.usage.* attributes and Laminar trace metadata.
• Keep rendering behavior unchanged by treating token usage as a non-rendered normalized event.
• Leave JSON-RPC app-server mode unchanged because it does not currently carry Centaur session trace context.

Notes

• Amp uses the same usage parser if it emits compatible usage fields. If Amp does not expose token usage, no span is exported rather than estimating.
• Telemetry export failures are logged but do not fail the user turn.

Validation

• cargo fmt
• cargo test in crates/harness-server