feat: add validate_claim_type and comprehensive validation test suite

[Mozez155]

closes #118

Add a new validate_claim_type function that rejects empty strings, claim
types longer than 64 characters, and any character outside [A-Za-z0-9_-].
Wire it into create_attestation, create_attestations_batch,
register_claim_type, and request_attestation.

Add InvalidClaimType = 28 to the Error enum.

Add a validation_tests module (30 tests) covering every validation function
with negative paths, boundary values, and no-state-change assertions:

• validate_claim_type: empty, >64 chars, space/dot/@, boundary at 64,
  underscore/hyphen allowed, via register_claim_type
• require_admin: wrong address, uninitialized contract, no state change
• require_issuer: unregistered, removed issuer, no state change
• validate_native_expiration: past, equal-to-now, future, None, no state change
• validate_metadata: 257-char rejected, 256-char accepted
• validate_tags: 6 tags rejected, tag >32 chars rejected
• validate_reason: 129-char rejected, 128-char accepted
• validate_import_timestamps: future timestamp, expiration before timestamp
• require_not_paused: paused contract rejects writes

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