fix(config): preserve quoted strings when stripping JSONC trailing commas

[March-7]

Summary

The trailing-comma regex also removed commas inside quoted JSONC strings. Settings such as memoryExtraParams.stop: [",}", ",]"] became ["}", "]"], and a storagePath containing the same punctuation changed during configuration loading.

Replace that regex with a string-aware scan that removes structural trailing commas while preserving quoted keys and values. Add parser regressions and a configuration-to-provider-parameters regression. No dependencies or configuration precedence changes.

Test plan

• Exercised the change locally: focused Bun parser/config suites go from 18 passed / 9 failed to 27 passed; independent real-file configuration cases go from 2/7 to 7/7. No model or HTTP calls.
• Compared 2,000 deterministic JSON/JSONC cases against jsonc-parser 3.3.1.
• Built both the unchanged baseline and patched checkout; full bun run check passes on both, and patched bun run typecheck passes.
• Noted limitations: complete bun test is not green in this environment. Baseline: 731 passed, 4 skipped, 32 failed (767 tests). Patched: 742 passed, 4 skipped, 32 failed (778 tests). All 32 failure IDs and diagnostics match after normalizing temporary directory names. Failures include child processes denied access to the real home directory and dependent lock-worker errors. This does not establish a passing full suite outside this environment.

The complete runs used the same original parent-process home adapter and model-download barrier; no inherited child preload options. An intermediate experimental runner had an argument-parsing error that recursively spawned Bun; it was stopped and excluded from the comparison. No production or upstream test changes were made to hide these environment failures.

Checklist

• Branch is based on main.
• bun test passes — the baseline-matched failures above remain.
• bun run typecheck passes.
• bun run check passes (format + lint + typecheck).
• Docs / README: not needed; existing JSONC configuration syntax and intended values are preserved.

AI assistance was used for implementation and testing, including review by a coordinating AI agent.

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