fix(runtime): reconcile partial usage observation

[fennoai]

Requested by @phantom5099

Summary

• avoid emitting non-nil usage when provider stream does not return token usage
• add input/output observed flags to provider usage payload and propagate to runtime stream handling
• reconcile token ledger per-side so partial observation keeps observed fields and only degrades missing side
• add regression tests for anthropic/gemini/openaicompat adapters and runtime ledger reconciliation