fix(python): normalize webhook signature hex comparison and handle case-insensitive headers

[Adityakk9031]

close #40

Description

Fixes issue #40 in the Python SDK (runinfra):

1. Case-Insensitive Webhook Signatures: verify_webhook_signature() generated a lowercase hex digest using hexdigest(), but compared it against incoming signatures directly with hmac.compare_digest(expected, signature). If an incoming webhook payload contained an uppercase hex signature string (e.g. v1=4F2A...), hmac.compare_digest failed due to case-sensitive character comparison. Updated to compare signature.lower() in constant time.
2. Case-Insensitive Content-Type Header Lookup: _Speech.create used dict.get("content-type", dict.get("Content-Type", ...)). Updated to perform case-insensitive header matching across response header keys.

Testing & Verification

• Added test_construct_webhook_event_accepts_uppercase_hex_signature to python/tests/test_runinfra_sdk.py.
• Ran full Python unit test suite: 159/159 tests passed (Ran 159 tests in 0.975s - OK).
• Ran node scripts/verify-version-sync.mjs: Version sync intact at 0.2.0.

[Adityakk9031]

@jaberjaber23 have a look