$ llm codec: cache-write and Cohere cached tokens bill at the input tier, as in 1.5.5

[MattJackson]

What. The OpenAI Chat and Responses readers moved prompt_tokens_details.cache_write_tokens / input_tokens_details.cache_write_tokens out of the input total onto the cache-write tier, and the Cohere reader moved usage.cached_tokens onto the cache-read tier. This restores 1.5.5: both stay inside the input count and bill at the input tier.

Why. Row-159 audit, plane-llm L3#2 (HIGH) and L2#4 (HIGH). 1.5.5 never split these: v1.5.5 crates/busbar/src/proto/openai_chat/reader.rs:693 (stream) and :966 (buffered), crates/busbar/src/proto/openai_responses/reader.rs:1022 and :1341, crates/busbar/src/proto/cohere/reader.rs:709-729 and :964-979 each set the split tier to None. Nothing signs the move. BUSBAR-1.6.0.md THE LLM DIALECT FIDELITY RULE says "Billing never moves". §7 Money and #77 say the money path stays 1.5.5's unless the owner signs a change, and #81 signs only fractional counts. CHANGELOG 1.6.0 Upgrade says "The token classes, their figures and every tokens: cap count exactly what 1.5.5 counted". LEDGER-100 is about adding units that were never ledgered. It does not cover these, because 1.5.5 already ledgered them, as input.

RED-able tests (the expected figures come from the 1.5.5 lines above):

• openai_chat/tests/unreadable_count_refusal_tests.rs::cache_write_tokens_bill_inside_the_input_on_every_path and openai_responses/...::cache_write_tokens_bill_inside_the_input_on_every_path: prompt 100, cache write 40 (also as "40"), output 9 ledgers input 100, output 9, no cache write, on the buffered, stream and truncated paths. Before this change: input 60 / cache write 40, and the string spelling was refused.
• The usage_census_tests for Chat and Responses class cache_write_tokens as a slice of the input. In the Cohere census, cached_tokens moves nothing.
• codec/tests/cache_tier_capture_tests.rs: the three dialects' capture tests, rewritten against the recorded bodies to pin the 1.5.5 tiers.

No ledger, oracle cell or ceiling changes. The oracle mock reports cache-write 0 and no Cohere cached_tokens, so no recorded cell moves.

[MattJackson]

row 155 B2 (#974) edits crates/busbar-plane-llm/src/codec/{cohere,openai_chat,openai_responses}/mod.rs at fn claims and the DECL quota_exceeded_status / auth_failure_status_and_kind rows; disjoint from this PR's hunks.