fix(auth): validate integer expiry before computing hmac signature in session_signer.py

[Vishaaallll]

Summary

Re-orders verification checks in session_signer.verify() in ods/extensions/services/dashboard-api/session_signer.py:

• Parses and validates that expiry_str is a valid integer BEFORE computing the HMAC-SHA256 signature (_sign(payload)).
• Returns (False, "malformed") immediately upon integer conversion failure.
• Adds a unit test in test_session_signer.py asserting _sign() is never invoked when an invalid expiry token is passed.

Why It Is Useful

• Optimizes Authentication Overhead: Avoids unnecessary CPU computation (hmac.new + SHA256 hashing) when processing invalid or corrupted session tokens.
• Mitigates Denial-of-Service Risk: Prevents attackers from forcing repetitive cryptographic calculations using junk expiry values.
• Defensive Execution Order: Ensures simple structural validation occurs prior to crypto operations.

Verification

• Ran test suite pytest ods/extensions/services/dashboard-api/tests/test_session_signer.py -v (21 passed).