fix(harness): don't re-stamp model metadata on a sandbox continuation

[pedrofrxncx]

Follows #6151, which added withModelMetadata to stamp metadata.models.thinking onto the claude-code harness's first start chunk (mirroring what Decopilot already does).

Bug: sandbox-dispatch-client.ts's dispatchOnce wraps every dispatch attempt — including sandbox-lost continuations — in withModelMetadata, and each call gets a fresh generator whose internal stamped flag resets to false. dispatchWithContinuation already drops a continuation's start chunk (documented: forwarding a second one would re-id the message), but it does NOT know about the message-metadata chunk withModelMetadata yields right after start — so a run that survives a sandbox-lost retry gets a second, redundant message-metadata chunk spliced into an already-open message. Harmless when the model id is unchanged, but it's dead weight on every continuation and a landmine if a retry ever picks a different model.

Fix: pass null for the model id on a continuation attempt (resume ? null : modelEnv.CLAUDE_CODE_MODEL) so withModelMetadata no-ops, consistent with dispatchWithContinuation already treating a continuation's start as a no-op.

Verify: bun test apps/api/src/harnesses/sandbox-dispatch-client.test.ts — added a regression test (a continuation does not re-stamp model metadata) composing dispatchWithContinuation + withModelMetadata the same way the real dispatch path does, asserting exactly one message-metadata chunk survives a sandbox-lost retry. Also ran apps/api/src/harnesses/with-model-metadata.test.ts (unchanged, still green) and cd apps/api && bunx tsc --noEmit.

Locally ran: bun run fmt, bunx tsc --noEmit (apps/api), the two targeted test files, bunx oxlint on both changed files. Full CI covers the rest.

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Summary by cubic

Prevent duplicate model metadata on sandbox continuation retries. Previously, a continuation emitted a second message-metadata chunk because dispatchOnce always wrapped attempts in withModelMetadata; downstream dropped the second start but not the metadata. Now, continuation attempts pass null as the model id so withModelMetadata no-ops, matching how the continuation’s start is suppressed and avoiding mismatched model ids.

• Changes: in sandbox-dispatch-client.ts, pass resume ? null : modelEnv.CLAUDE_CODE_MODEL into withModelMetadata.
• Behavior: first attempt still stamps metadata; continuations no longer add another message-metadata chunk.
• Tests: adds a regression test asserting exactly one message-metadata survives a sandbox-lost retry.

^{Written for commit 5d27e27. Summary will update on new commits.}