Align Anthropic usage accounting with provider contract

[elrensmin]

Summary

Align Anthropic streaming usage normalization with the provider's response contract.
Anthropic's input_tokens already excludes tokens read from or written to the
prompt cache, so Tau preserves it as fresh input and reports total input as the
sum of fresh, cache-read, and cache-write tokens.

Changes

• Preserve Anthropic's provider-reported input_tokens in message_start and
  message_delta.
• Recompute totals from fresh input, cache reads, cache writes, and output.
• Keep 1-hour cache-write details and ignore malformed or negative counters.
• Add independent coverage for delta ordering, partial/repeated updates,
  delta-only usage, zero/null values, and malformed values.

Validation

• uv run pytest — 1879 passed, 3 skipped
• uv run ruff check .
• uv run ruff format --check .
• uv run mypy
• Hugo and Pagefind website builds