fix: normalize Windows project file paths

[VIVEHACKER]

Motivation

• Server-side flows that inspect or write project files were failing when snapshots or inputs used Windows-style absolute paths (drive letters/backslashes), causing downstream operations like computing diffs or detecting package.json to behave incorrectly.
• Normalize and safely resolve repo-relative paths so preview, sandbox, and session logic work consistently for projects originating from Windows checkouts and cloud (Linux) runners.

Description

• Add apps/platform-api/src/common/file-path.utils.ts providing normalizeProjectFilePath, resolveProjectFilePath, and matchesProjectFilePath utilities to normalize Windows drive prefixes, convert backslashes, and perform tolerant suffix-based matching.
• Update preview flow (apps/platform-api/src/modules/preview/preview.service.ts) to detect package.json via matchesProjectFilePath and to write files using resolveProjectFilePath.
• Update sandbox build flow (apps/platform-api/src/modules/sandbox/sandbox.service.ts) to detect package.json/tsconfig* with matchesProjectFilePath and to write files using resolveProjectFilePath prior to install/build steps.
• Update session framework detection (apps/platform-api/src/modules/session/session.service.ts) to normalize incoming file paths with normalizeProjectFilePath and to locate package.json using matchesProjectFilePath.

Testing

• Ran a targeted runtime check with pnpm --filter @ai-platform/platform-api exec ts-node --transpile-only -e "...file-path.utils" which printed normalized examples and validated that a Windows-style path matches package.json, and the check succeeded.
• Attempted pnpm --filter @ai-platform/platform-api build to validate the service build, but the workspace build failed due to pre-existing TypeScript/Prisma typing errors unrelated to the path-normalization changes.

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[chatgpt-codex-connector]

Drop the preserved absolute path prefix before joining

When a snapshot contains an absolute file like C:\repo\package.json or /home/alice/repo/package.json, resolveProjectFilePath() writes it to <baseDir>/repo/package.json instead of <baseDir>/package.json. matchesProjectFilePath() still treats that same entry as the root manifest, so startPreview() and runBuild() invoke npm install from the workspace root (apps/platform-api/src/modules/preview/preview.service.ts:75-84, apps/platform-api/src/modules/sandbox/sandbox.service.ts:56-67), where no package.json exists. In the absolute-path cases this patch is meant to support, preview/build still fail.

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