fix(common): improve path validation and tilde expansion

[echobt]

Summary

Fixes #5181 and #5183 - Path validation and expansion issues.

Problems

1. validate_path_safety false positive on '..' in filenames
2. expand_tilde doesn't handle standalone '~'

Solution

1. Check path components instead of substring match
2. Handle bare '~' as valid home directory reference

[greptile-apps]

Greptile Overview

Greptile Summary

This PR delivers multiple critical security and reliability fixes across path handling, command validation, concurrency control, and resource management.

Key Changes:

• Path Validation (paths.rs): Fixed false positive in validate_path_safety that incorrectly flagged filenames containing ".." (like file..txt) as path traversal attempts by checking path components instead of substring matching
• Tilde Expansion (paths.rs): Added support for bare ~ to properly expand to home directory (previously returned unchanged)
• Command Bypass Prevention (validation.rs): Introduced normalize_command() function to prevent validation bypass attempts through extra whitespace, quotes, or path variants (e.g., /bin/rm, 'rm')
• TOCTOU Race Fixes: Eliminated time-of-check-to-time-of-use races in MCP server initialization (server.rs) and plugin registration (registry.rs) by holding write locks during entire check-and-set operations
• Bounded Response Storage (response_store.rs): New module providing bounded tool response storage with max capacity (500 entries), TTL-based cleanup (5min), and automatic eviction to prevent unbounded memory growth

Testing: All changes include comprehensive unit tests covering edge cases and security scenarios.

Confidence Score: 5/5

• This PR is safe to merge with high confidence - fixes are well-designed, thoroughly tested, and address real security vulnerabilities
• All changes are defensive security improvements with excellent test coverage. The path validation fix eliminates false positives while maintaining security. Command normalization closes bypass vectors. TOCTOU race fixes use standard Rust patterns. The new response store prevents memory leaks with bounded storage.
• No files require special attention - all changes are straightforward security improvements

Important Files Changed

Filename	Overview
src/cortex-cli/src/utils/paths.rs	Fixed path traversal false positives and tilde expansion for bare ~, with comprehensive tests
src/cortex-engine/src/validation.rs	Added command normalization to prevent validation bypass via whitespace/quotes/paths
src/cortex-engine/src/tools/response_store.rs	New bounded tool response store with TTL-based cleanup and eviction policies
src/cortex-mcp-server/src/server.rs	Fixed TOCTOU race in initialize by holding write lock during check-and-set
src/cortex-plugins/src/registry.rs	Fixed TOCTOU race in plugin registration using HashMap entry API

Sequence Diagram

sequenceDiagram
    participant User
    participant PathValidator
    participant CommandValidator
    participant MCPServer
    participant PluginRegistry
    participant ResponseStore

    Note over PathValidator: Path Validation Fix
    User->>PathValidator: validate_path("file..txt")
    PathValidator->>PathValidator: Check path components (not substring)
    PathValidator-->>User: ✓ Valid (not traversal)
    
    User->>PathValidator: expand_tilde("~")
    PathValidator->>PathValidator: Handle bare tilde case
    PathValidator-->>User: /home/user

    Note over CommandValidator: Command Bypass Prevention
    User->>CommandValidator: validate("/bin/rm -rf /")
    CommandValidator->>CommandValidator: normalize_command()
    CommandValidator->>CommandValidator: Extract basename: "rm"
    CommandValidator->>CommandValidator: Check against blocked: "rm -rf /"
    CommandValidator-->>User: ✗ Blocked

    Note over MCPServer,PluginRegistry: TOCTOU Race Fixes
    User->>MCPServer: initialize()
    MCPServer->>MCPServer: Acquire write lock
    MCPServer->>MCPServer: Check state == Uninitialized
    MCPServer->>MCPServer: Set state = Initializing
    MCPServer->>MCPServer: Release write lock
    MCPServer-->>User: ✓ Initialized

    User->>PluginRegistry: register(plugin)
    PluginRegistry->>PluginRegistry: Acquire write lock
    PluginRegistry->>PluginRegistry: Entry API check-and-insert
    PluginRegistry-->>User: ✓ Registered

    Note over ResponseStore: Bounded Storage
    User->>ResponseStore: store(response)
    ResponseStore->>ResponseStore: Check capacity (500)
    alt At capacity
        ResponseStore->>ResponseStore: Evict oldest entry
    end
    ResponseStore->>ResponseStore: Store new response
    ResponseStore-->>User: ✓ Stored
    
    User->>ResponseStore: take(call_id)
    ResponseStore->>ResponseStore: Remove entry (cleanup)
    ResponseStore-->>User: Response

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[greptile-apps]

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[echobt]

Consolidated into #80 - fix: consolidated memory and storage improvements