Add a realistic preview mock filesystem and file RPC support

[Copilot]

The preview environment had no useful filesystem model, which limited FileSystem integration work and made file-oriented previews hard to exercise. This change adds a realistic mock filesystem rooted at /Users/mike and teaches mockwaveenv to answer the core file RPCs used by preview flows.

• Mock filesystem dataset

  • Adds a dedicated mock filesystem module for preview usage.
  • Seeds a macOS-style tree rooted at /Users/mike with 100+ file entries across common directories like Documents, Downloads, Pictures, Projects, and waveterm.
  • Includes a small set of files with actual payloads so preview consumers can render meaningful content:
    • text (.bashrc)
    • markdown (meeting-notes.md, repo docs)
    • images (.png, .jpg)

• Mock file RPC implementations

  • Wires preview-backed implementations for:
    • FileInfoCommand
    • FileReadCommand
    • FileListCommand
    • FileJoinCommand
    • FileListStreamCommand
    • FileReadStreamCommand
  • Supports:
    • wsh://... URIs and plain paths
    • path normalization (~, relative paths, . / ..)
    • directory reads returning entries
    • hidden file filtering via FileListOpts.all
    • offset/size range reads
    • streamed directory batches and file chunks

• Preview behavior improvements

  • Makes directory previews behave more like a real local filesystem.
  • Makes markdown-relative asset resolution via FileJoinCommand work against a coherent tree.
  • Gives future FileSystem integration work a stable preview substrate instead of ad hoc RPC stubs.

• Focused coverage

  • Adds preview mock tests for:
    • filesystem population shape
    • file metadata lookup
    • file reads
    • directory listing
    • path joining
    • stream responses

Example:

const joined = await env.rpc.FileJoinCommand(null as any, [
    "wsh://local//Users/mike/Documents",
    "../waveterm/docs",
    "preview-notes.md",
]);

const file = await env.rpc.FileReadCommand(null as any, {
    info: { path: "/Users/mike/.bashrc" },
});

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[cloudflare-workers-and-pages]

Deploying waveterm with    Cloudflare Pages

Latest commit:	2778364
Status:	✅  Deploy successful!
Preview URL:	https://52c6d485.waveterm.pages.dev
Branch Preview URL:	https://copilot-improve-preview-mock.waveterm.pages.dev

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[kilo-code-bot]

Code Review Summary

Status: No Issues Found | Recommendation: Merge

Files Reviewed (3 files)

• frontend/preview/mock/mockfilesystem.ts - New mock filesystem implementation for testing
• frontend/preview/mock/mockwaveenv.test.ts - Added tests for mock filesystem
• frontend/preview/mock/mockwaveenv.ts - Added file-related RPC command handlers

The changes implement a mock filesystem for testing purposes, including:

1. New mock filesystem (mockfilesystem.ts): A comprehensive mock filesystem implementation that provides file info, read, list, join, and streaming operations for testing.

2. Test coverage (mockwaveenv.test.ts): Tests verifying the mock filesystem is properly rooted at /Users/mike and has at least 100 files and 10 directories, along with tests for all file operations.

3. RPC handlers (mockwaveenv.ts): Added handlers for fileinfo, fileread, filelist, filejoin, filereadstream, and fileliststream commands using the new mock filesystem.

The implementation is well-structured with proper async generator support for streaming operations.

Verification performed: Reviewed all changed files and confirmed no issues found.