feat(vscode): insert dropped resource paths

[ousamabenyounes]

Closes #2245.

What changed

• Decode VS Code Explorer's standard URI-list drag payload and insert dropped files or folders as chat mentions at the current caret.
• Use paths relative to the active working directory, while retaining absolute paths for external resources.
• Support POSIX, Windows drive, and UNC-style paths, including canonical quoting for paths containing spaces.
• Accept Remote SSH, WSL, dev-container, and other virtual-workspace resources only when their scheme and authority match the active workspace.
• Preserve the existing image/video drop behavior by giving media uploads precedence when a drop exposes both files and URI data.

Why

The webview already intercepted every document drop, but only consumed media files. Explorer resources therefore appeared to be accepted while their paths were silently discarded. This adds the missing resource-path branch without regressing media uploads.

Test verification (RED → GREEN)

• RED: the new focused regression suite failed before implementation because the drop-path module did not exist.
• GREEN: focused suite: 11 tests passed, including remote workspaces, nested workdirs, and authority rejection.
• GREEN: VS Code extension suite: 16 files / 316 tests passed.
• GREEN: full local suite: lint, typecheck, 1,036 test files / 16,908 tests, and CLI build passed versus the upstream baseline of 1,035 files / 16,897 tests.
• Revert proof: removing the implementation reproduces the original missing-module RED in the focused regression suite.
• Independent code review: PASS after correcting media-drop precedence and canonical quoted mentions.

Disclosure

AI-assisted using OpenAI Codex. The human contributor reviewed the implementation and remains responsible for understanding, testing, licensing, and maintaining every submitted line.

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[ousamabenyounes]

Addressed the remote-workspace review in e8b4519ee: Explorer drops now accept only the active virtual workspace's exact scheme/authority, reject credential or cross-host payloads, and stay relative to a nested active workdir.

Validation: reproduced vscode-remote: being discarded and the authority bypass as RED; focused tests are 11/11 GREEN, and run-ci.sh improved from the upstream baseline of 16,897 passing tests to 16,908 with lint, typecheck, and build GREEN.