refactor(coderd/x/chatd/chatsanitize): drop Anthropic guard escalation passes - #30970

ApplyAnthropicProviderToolGuard now validates the result of its targeted repair once, then returns it or fails closed. This removes 163 production lines (2 added): the escalation that ran after a failed repair (an all-messages repair, then two strip passes) and its helpers isSafeAnthropicProviderToolPrompt, providerExecutedToolMessageIndexes, stripAnthropicProviderToolHistoryFromMessages and addAnthropicProviderToolSanitizationStats, plus their internal tests. git grep finds no other callers.

Why it is safe

The escalation ran but could never repair a prompt. SanitizeAnthropicProviderToolHistory loops until nothing is left to remove outside the last assistant message when that message carries signed or redacted reasoning, so after the targeted repair any violation left is in that message. Every later pass skips that message, so the guard failed anyway.

• A differential test (not committed) ran the old and new guard on 1,000,000 random prompts and on every prompt the existing chatsanitize, chatloop and chatd Anthropic tests pass in. Outputs and errors matched in 1,000,000 of 1,000,000. 200,020 of 1,000,000 prompts reached the escalation; 0 came out repaired.
• New subtest invalid pair in latest signed assistant fails closed pins the failure path (nil prompt, ErrAnthropicProviderToolPromptUnsafe, log fields). It passes before and after; no other test covered that path.
• New sanitizer row drops provider result without output text takes over the one deleted assertion no other test made: it kills the mutant that keeps an empty-output provider result as empty text. The log detail cap test stays. All 12 mutants of the code the deleted tests touched fail a remaining test.

Review focus

Failures log less: the fallback and strip warnings are gone, and the error log drops immutable_latest_signed_assistant (always true), no longer says "nuclear strip", and its message_index values point into the prompt the guard validated.

Validation

• go test ./coderd/x/chatd/chatsanitize/... ./coderd/x/chatd/chatloop/... -count=1, go test ./coderd/x/chatd/ -run 'Anthropic|Guard|Sanitiz' -count=1 -p 4 -parallel 4, go vet, golangci-lint on chatsanitize: pass.

Stack

This stack moves the Anthropic guard unit test into chatsanitize, then deletes the guard escalation that can never repair a prompt.

1. test(coderd/x/chatd): move the Anthropic guard unit test into its own file #30968 Move the Anthropic guard unit test into its own file
2. test(coderd/x/chatd): move the Anthropic guard unit test into chatsanitize #30969 Move the Anthropic guard unit test into chatsanitize
3. refactor(coderd/x/chatd/chatsanitize): drop Anthropic guard escalation passes #30970 Drop the Anthropic guard escalation passes

Xum, an AI agent, opened this PR for @ibetitsmike and wrote this description.