fix: repair Claude API requests and report rejections accurately

[jvsena42]

Summary

Investigating a reported HTTP 400 from the Claude API turned up three separate defects. The immediate cause was an exhausted credit balance, but the app reported it as Unexpected error (400). Please try again. — advice that cannot work, since retrying never restores credit. Fixing the message then revealed two further bugs that the billing failure had been masking: every request we send is malformed, and the model we target no longer exists.

Reproduced end-to-end on device via the Gaps tab (GapsViewModel:49 sets isAiMode = hasKey and loads on entry, so it issues a real request with no photo needed):

[ClaudeApi] HTTP 400 body={"type":"error","error":{"type":"invalid_request_error",
"message":"Your credit balance is too low to access the Anthropic API.
Please go to Plans & Billing to upgrade or purchase credits."}}

Topping up credits is still required to actually use the AI features — that part is account-side and outside this PR. These changes ensure that once billing is resolved the requests are valid, and that any future rejection explains itself.

Changes

• Serialise the full image source. Json defaults to encodeDefaults = false, so ClaudeImageSource's type and media_type defaults were silently dropped and image blocks went out as bare {"data":"..."}. explicitNulls = false is set alongside it so the unused member of a content-block union is omitted rather than sent as an explicit null.
• Move off a retired model. claude-sonnet-4-20250514 was retired on 2026-06-15; it no longer resolves. Now claude-sonnet-5.
• Disable thinking. Thinking is on by default from Sonnet 5 onward and shares the max_tokens budget with the response. These call sites parse the reply as JSON, so a truncated body breaks decoding — disabling it keeps the full 1024 tokens for the answer.
• Relay the API's own reason for a 400. Puts the already-defined but unused ClaudeErrorResponse to work; falls back to the status code when the body will not parse. 401/429/5xx branches are unchanged.
• Log error bodies, mirroring YouCamApiClient.ensureSuccess — nothing previously recorded why a request failed. The body carries no credentials.
• Add ClaudeApiClientTest (5 tests) asserting the serialised request shape and the error-message mapping.

Test plan

• ./gradlew :shared:testAndroidHostTest — 5 new tests, all passing. They were written against the unmodified client first and failed as expected, confirming each defect was real rather than assumed; the pre-fix body was "source":{"data":"AQID"} with no type or media_type.
• ./gradlew detekt — passes.
• On device: opened the Gaps tab and confirmed the screen now shows "Your credit balance is too low to access the Anthropic API. Please go to Plans & Billing to upgrade or purchase credits." in place of the previous Unexpected error (400). Please try again.
• The malformed-payload and retired-model fixes could not be verified against the live API, since the credit-balance failure is rejected before either is evaluated. They are covered by unit tests only, and warrant a re-check on device once credits are added.

Checklist

• ./gradlew detekt passes
• Tested on Android
• Tested on iOS
• Updated documentation (if applicable)

Notes for review

• Shared-module code only — no UI was touched, so the usual "update both Android and iOS" rule does not apply here. iOS consumes the same ClaudeApiClient and picks these fixes up unchanged; it is left unchecked above only because it was not run.
• Relayed messages are English regardless of app locale, unlike the hardcoded strings around them. Localising properly means plumbing error types through to the UI rather than strings — a larger refactor, deliberately left as follow-up. The tradeoff seemed worth it: an accurate English message beats a localised but actively wrong one.

🤖 Generated with Claude Code