[WRONG BRANCH] fix(bridge): decode freeform previews incrementally

[luvs01]

Motivation

• Freeform tool preview handling reparsed the entire accumulated argument buffer on every streamed chunk, creating quadratic CPU and allocation work for many small chunks and enabling an availability DoS against the local proxy.

Description

• Replace full-buffer reparse with an incremental stateful preview decoder that tracks previewMode, prefix data, escape state, and pending \uXXXX digits per open tool call and decodes only newly arrived characters.
• Preserve behavior for the three preview modes (prefix → wrapped → raw) and continue to emit response.custom_tool_call_input.delta for decoded input without rewinding on mode flips.
• Remove the previous full-buffer freeformPartialInput reparsing path and add a focused regression test that streams a 32,000-character freeform input one byte at a time to verify incremental decoding.

Testing

• Ran bun run typecheck and it passed with no type errors.
• Ran bun test tests/bridge.test.ts and the focused bridge suite passed, including the new large one-byte-stream regression (56 tests total passed).
• Ran bun run privacy:scan and it passed.
• Attempted a full bun run test run but earlier runs of the full suite experienced unrelated management/OAuth test timeouts and failures, so the full repository-wide suite was not relied on to validate this focused fix.

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📥 Commits

Reviewing files that changed from the base of the PR and between 57140d6 and 2232fbd.

📒 Files selected for processing (2)

• src/bridge.ts
• tests/bridge.test.ts

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[github-actions]

✅ Deterministic PR hygiene checks passed.

[github-actions]

⏳ DRAFT

• wrong target branch (main); retarget to dev.

What to do

• Retarget this PR to dev — all contributions go to dev.

Its title has been prefixed with [WRONG BRANCH].
This pull request is being kept as a draft automatically. Once every issue above is resolved, it will be marked ready for review again.