test(agent): add test coverage for loadSecret

[Fathia-tech14]

Resolves #817

Summary of Changes

• Exported loadSecret from agent/agent.js.
• Added a comprehensive unit test suite in agent/loadSecret.test.js (30 tests) testing:
  • Environment Variable Secret (AGENT_STELLAR_SECRET): Valid string return and character preservation.
  • Configuration Validation & Conflicts: Proper errors when neither or both variables are set.
  • File Secret (AGENT_STELLAR_SECRET_FILE):
    • File reading failures (ENOENT, EACCES).
    • Whitespace, newline, and carriage return trimming.
    • POSIX permission checking and numeric bitmask boundaries (0o040 group read and 0o004 other read boundaries).
    • Safe modes permitted (0o600, 0o400, 0o700, 0o620, etc.).
    • Insecure modes rejected (0o640, 0o604, 0o644, 0o666, 0o777, etc.).
    • Windows platform (win32) permission bypass behavior.
  • Isolation & Mocking: Network and contract calls remain uninvoked, and process.env / process.platform are restored after every test.

Verification

Ran npx vitest run in agent/:

✓ runSummary.test.js (20 tests)
✓ loadSecret.test.js (30 tests)
✓ agent.test.js (30 tests)

Test Files  3 passed (3)
     Tests  80 passed (80)

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[Fathia-tech14]

@Stellar-Ecosystem Hi! This PR is open and ready for review — happy to address any feedback. Thanks!