Fix: Secondary Security Vulnerabilities (#138)

[HarshavardhanVemali]

Resolves #138.

This PR introduces fixes for the secondary security vulnerabilities reported:

1. RCE via eval(): Validated input using strict regular expressions in calculator_tools.py and claude_function_agent.py to mitigate arbitrary code execution risks.
2. Insecure Temp Directory: Replaced hardcoded /tmp with tempfile.gettempdir() in policy.py and tests.
3. Protocol Injection: Replaced urllib.request.urlopen with the requests library in call.py and sheets.py to mitigate protocol injection risks.

Testing completed:

• Syntax verified successfully across all modified Python files.
• Bandit confirms that B307, B108, and B310 vulnerabilities are resolved in the respective files.

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GSSoC Labels Requested:

• level:advanced
• type:bug
• type:security
• quality:clean

[HarshavardhanVemali]

Hello Mentors / Maintainers! As per the GSSoC contribution guidelines, could you please review this PR and add the appropriate labels?

Suggested labels:

• level:advanced
• type:bug
• type:security

Once reviewed, please also consider adding the gssoc:approved and mentor:username labels so the contribution counts towards GSSoC. Thank you!