fix(ai): include native Anthropic OAuth billing fingerprint

[chupe]

Summary

Adds the missing billing fingerprint to native Anthropic OAuth requests, together with the external CLI user-agent suffix and Agent SDK identity. The fingerprint uses the first serialized user text block and browser-safe SHA-256. The existing supported client version remains 2.1.251.

This is separate from the version-floor fix in #1289 and the stale version fixed in pi-anthropic-auth #60. The related OMO #3892 targeted OpenCode and closed without merging.

Behavior

• Native OAuth requests include deterministic cc_version, cc_entrypoint=sdk-cli, and cch fields before payload hooks run.
• Caller system blocks, cache checkpoints, metadata, and forced-tool-choice fallback retain their behavior.
• Ordinary API-key, Cloudflare, and Copilot requests do not gain OAuth fields.
• No dependency, lockfile, authentication-storage, or version-pin changes.

Verification

Check	Result
New final-fetch regressions before the fix	14 expected failures, 3 adjacent passes
Focused OAuth suites	36 passed
Anthropic provider group	216 passed, 39 existing opt-in live cases skipped
Root bun run check	Passed, including compiler and browser smoke
Telemetry build and AI build:offline	Passed
Real source CLI, isolated OAuth fixture against localhost	Exit 0; fingerprint hashes matched the serialized input; final marker returned
Real source CLI, isolated API-key fixture	Exit 0; no billing block; caller system input preserved
Isolation	Both sandboxes removed; real auth unchanged
Independent gate review	APPROVE; eight real-SDK hash cases independently reproduced, including Unicode

Exact commands and sanitized receipts are retained locally under local-ignore/qa-evidence/20260905-oauth-upstream/ (red.txt, green.txt, cli-qa.json, README.md), excluded from Git per repository policy. The CLI proof uses the repository's Senpi QA sandbox and fake-model helpers.

Scope and risk

The change is confined to the native OAuth branch. Fingerprint input edge cases, payload-hook preservation, browser imports, and non-OAuth paths have regression coverage. No real provider credentials were used for these checks. LSP could not initialize with the installed TypeScript 7 tooling; the actual repository compiler passed.

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Summary by cubic

Fixes native Anthropic OAuth requests being rejected for missing the deterministic billing fingerprint and external CLI/Agent SDK identity.

Adds a x-anthropic-billing-header system block (cc_version, cc_entrypoint=sdk-cli, cch) derived from the first serialized user text block via browser-safe SHA-256, plus the (external, cli) user-agent suffix and Agent SDK identity. The block is prepended before payload hooks run, and only the native OAuth branch is affected; API-key, Cloudflare, and Copilot requests stay unchanged.

Bug Fixes

• Preserves caller system blocks, cache checkpoints, metadata, and forced-tool-choice fallback behavior.
• Keeps the supported Claude Code version at 2.1.251 with no dependency, lockfile, or auth-storage changes.
• Adds regression coverage for fingerprint edge cases (string, text blocks, image-only, empty input) and all non-OAuth paths.

^{Written for commit 770262f. Summary will update on new commits.}