security: harden shellQuote and consolidate shell escaping in gcp.ts

[la14-1]

Why: Eliminates inconsistent shell-escaping patterns in gcp.ts that could allow a null-byte truncation attack to bypass quoting, and reduces future regression risk by consolidating all escaping through a single hardened function.

Fixes #2529

Changes

• Hardened shellQuote: Added null-byte rejection as defense-in-depth (callers already validate, but the function itself should be safe to call independently)
• Exported shellQuote: Makes it testable and available for consistent use across GCP module
• Consolidated interactiveSession: Replaced inline cmd.replace(/'/g, "'\\''") with shellQuote(cmd) — eliminates duplicate escaping logic that could drift out of sync
• Added test suite: 12 tests covering shell metacharacters, null bytes, single-quote escaping, empty strings, newlines, tabs, backslashes, and consecutive quotes

Security Analysis

The shellQuote function itself uses the correct POSIX single-quote escaping technique ('\''), which is the same approach as Python's shlex.quote(). The real risks addressed:

1. Null-byte truncation: A null byte in the input could cause the C-level string to terminate early, potentially leaving shell metacharacters unquoted. Now rejected at the shellQuote level.
2. Inconsistent escaping: interactiveSession was doing inline replace() instead of calling shellQuote, meaning any future hardening to shellQuote wouldn't propagate. Now consolidated.

Test plan

• bun test src/__tests__/gcp-shellquote.test.ts — 12/12 pass
• bun test — 1408/1408 pass (0 regressions)
• bunx @biomejs/biome check src/ — 0 errors

-- refactor/security-auditor

[louisgv]

Security Review

Verdict: APPROVED ✅
Commit: eea7e9e

Summary

This PR significantly improves command injection protection in GCP SSH session handling by hardening and consolidating shell escaping through a dedicated shellQuote() function.

Security Findings

No vulnerabilities found. This PR introduces strong security improvements:

1. Enhanced Command Injection Protection (HIGH IMPACT)

   • Consolidates shell escaping through hardened shellQuote() function (line 1026)
   • POSIX single-quote escaping ('\'' technique) prevents all shell metacharacter expansion
   • Protects against: $(), \`, ${}, |, ;, &&`, and other injection vectors

2. Defense-in-Depth: Null Byte Validation (MEDIUM IMPACT)

   • Added null byte check directly in shellQuote() (line 1092-1094)
   • Complements existing validation in interactiveSession() (line 1020)
   • Protects against C-level string truncation attacks

3. Comprehensive Test Coverage (HIGH VALUE)

   • 12 new tests covering all critical security scenarios
   • Validates protection against command substitution, variable expansion, shell operators
   • Tests edge cases: empty strings, consecutive quotes, newlines, tabs, backslashes
   • Confirms null byte rejection

Tests

• ✅ bun test: 1400 pass / 0 fail (including 12 new shellQuote tests)
• ✅ biome lint: 0 errors
• ✅ macOS compat: POSIX single-quote escaping is portable
• ✅ curl|bash: N/A (TypeScript code, not shell script)

Version Management

• ✅ Version correctly bumped: 0.16.16 → 0.16.17

Code Quality

• Exported shellQuote enables reuse and testing
• Clear JSDoc documentation
• No regressions introduced

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-- security/pr-reviewer