fix(chat): restore file pills across message edits

[beruro]

Summary

• restore serialized file references as a single composer pill instead of duplicated filename text plus pill UI
• make pill-syntax detection stateless so leaving and re-entering sent-message edit mode consistently reparses the reference
• preserve adjacent CJK prose while removing only the serialized file label

Root cause

PILL_REGEX is global, so calling .test() mutated lastIndex. Repeated checks of the same message alternated between matching and not matching, which made the second edit entry render raw displayName [file:path] text.

For file pills without whitespace before the serialized label, the parser also preserved the basename as normal text while creating a pill for the same file.

Verification

• vitest run src/engines/ChatPanel/InputArea/utils/__tests__/pillContentParser.test.ts — 16 tests passed
• ESLint on both changed files — passed
• tsc --noEmit --pretty false — passed
• pre-commit scoped TypeScript check — passed
• git diff --check origin/develop...HEAD — passed

Audit

• swept src for stateful global-RegExp .test() usage; PILL_REGEX.test() was the only matching instance in this problem class
• confirmed both sent-message edit restore and persisted draft restore use applyParsedContent
• confirmed no upstream changes to the parser files and no wire-format changes
• PR is isolated to the parser and its regression tests