fix(harness): discard the delta coalescer's held text on a provider-retry restart

[pedrofrxncx]

Follows #6664 (token-level streaming, merged today), hardening the retry path it added.

On a transient provider rejection, runClaudeCode restarts the attempt and clears pending (pending.length = 0) since the dead attempt's buffered chunks aren't part of the next one. The createDeltaCoalescer added in the same PR can independently be holding an un-flushed text/reasoning delta at that exact point (deltas can accumulate before the turn's first assistant message, i.e. before started flips — the same window canRestartCleanly() checks) — nothing cleared it. A later drain() on the new attempt would then splice the dead attempt's partial, abandoned text into the new attempt's stream, or worse, merge it into an unrelated delta if the new attempt reuses the same block id.

Failure scenario: provider rejects the request mid-stream after emitting a partial (<200 char) text delta that never got flushed; the retry restarts a fresh SDK session, and the abandoned delta text reappears prefixed onto (or merged into) the first delta of the successful retry.

Fix: added discard() to the coalescer (drops the held delta without emitting it) and call it right next to pending.length = 0 in the transient-provider-rejection retry branch.

Regression test: discard drops what is held instead of emitting it in packages/harness-runner/claude-code.test.ts — pushes a delta, discards, drains (expects empty), then verifies a fresh push after discard is not merged with the discarded text.

To confirm: bun test packages/harness-runner/claude-code.test.ts.

Locally ran: bun run fmt, bunx tsc --noEmit (packages/harness-runner), the targeted test file above (39 pass), and bunx oxlint on both changed files (0 warnings/errors). Full CI validates the rest.

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Summary by cubic

Fixes the delta coalescer retaining abandoned text across provider-retry restarts, which could splice a dead attempt's partial delta into the new attempt's stream or merge it with an unrelated delta.

• Adds discard() to createDeltaCoalescer and calls it alongside the pending reset during retry.
• Includes a regression test verifying discarded text is not emitted or merged.

^{Written for commit e496ce8. Summary will update on new commits.}