feat(anthropic): add Claude 5 family pricing & align current rates

[KennethWKZ]

Summary

Adds Claude 5 generation metadata and aligns the Anthropic fallback pricing / context-limit tables in headroom/providers/anthropic.py with current Anthropic pricing (verified 2026-07-04). Several Claude 4.x entries carried stale rates, the new Claude 5 models (Fable 5, Opus 4.8, Sonnet 5) had no metadata, and the generic fallback tests disagreed with the provider tests.

Supersedes #1485 (rebased onto latest main, squashed to one commit, extended with Sonnet 5 / Fable 5 and the requested fallback-test fixes).

Changes

All values $ / MTok; cached_input = prompt-cache read = 0.1× input.

Tier	Model	Before	After	Context
Fable	claude-fable-5	— (new)	$10 / $50 / $1.00	1M
Opus	claude-opus-4-8	— (new)	$5 / $25 / $0.50	1M
Opus	claude-opus-4-7	$15 / $75 / $1.50	$5 / $25 / $0.50	1M
Opus	claude-opus-4-6	$15 / $75 / $1.50	$5 / $25 / $0.50	1M
Opus	claude-opus-4-5-20251101	$15 / $75 / $1.50	$5 / $25 / $0.50	200K
Sonnet	claude-sonnet-5	— (new)	$3 / $15 / $0.30	1M
Sonnet	claude-sonnet-4-6	— (new)	$3 / $15 / $0.30	1M
Sonnet	claude-sonnet-4-5	— (new)	$3 / $15 / $0.30	200K
Haiku	claude-haiku-4-5-20251001	$0.80 / $4 / $0.08 (3.5 rates)	$1 / $5 / $0.10	200K

claude-sonnet-4-20250514 and all Claude 3.x / 3.5.x entries were already correct — left unchanged.

The 1M-context entries (Fable 5, Opus 4.8, Sonnet 5, Sonnet 4.6) are functional, not cosmetic: they ship in the long-context tier, and without explicit entries the sonnet / opus pattern defaults would report 200K.

Sonnet 5 is pinned to the standard Sonnet tier ($3 / $15 / $0.30); Anthropic's introductory rate ($2 / $10 through Aug 31 2026) is intentionally not encoded to avoid a time-dependent fixture.

Fallback-model tests (addresses review on #1485)

_PATTERN_DEFAULTS["opus"] is aligned to the current Opus tier ($5 / $25 / $0.50) so the generic fallback suite and the provider-specific suite agree:

• test_pricing_for_known_models — Opus 4.5 pins $5 / $25 / $0.50
• test_pattern_based_inference_opus — unknown-opus fallback now $5 / $25
• test_cost_estimation_for_new_models — fixture estimate corrected $22.5 → $7.5
• test_pattern_based_inference_sonnet — retargeted to claude-sonnet-6-* (the old claude-sonnet-5-* probe now prefix-matches the real claude-sonnet-5 key)

New provider coverage:

• test_get_context_limit_claude_5_family — Fable 5 / Opus 4.8 / Sonnet 5 all 1M
• test_pricing_claude_5_family — exact rate table for the 3 new models

Full suite: 48 passed.

Source

https://platform.claude.com/docs/en/about-claude/pricing

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