Add tests for RSA key hashing, chain-model validity, and PEM explanatory text

[Xor-el]

Summary

Adds regression test coverage for three existing behaviours that were previously
unguarded. No production code changes: in each case the library already behaves
correctly, so these lock in the contract.

What's covered

RSA key parameter hashing (TRsaKeyParametersTest, new)

TRsaKeyParameters.GetHashCode must fold in both the modulus and the exponent.
Three cases:

• keys differing only in the exponent hash differently (and are unequal)
• keys differing only in the modulus hash differently (and are unequal)
• equal keys hash equally

Uses real RSA test-vector moduli, since TRsaKeyParameters validates its modulus.

Certification-path validity models (TValidityModelTest, new)

TPkixCertPathValidator honours PkixParameters.Date under both validity models:

• Shell model (PkixValidityModel): every certificate is checked against the
  validation date (a chain passes inside the end-entity window and fails before or
  after it).
• Chain model (ChainValidityModel): only the end-entity is checked against the
  date; each CA is checked at the time its subordinate was issued, so an intermediate
  that has since expired still validates.
• dateOfCertGen: the ISIS-MTT id-isismtt-at-dateOfCertGen extension
  (1.3.36.8.3.1) overrides the subordinate NotBefore when timing the issuer's check.

PEM explanatory text (TPemReaderTest.TestExplanatoryTextAroundObjects, new method)

Per RFC 7468 sec. 5.2, tools such as openssl pkcs7 -print_certs surround each object
with explanatory text. The reader must ignore it before the first BEGIN, between
objects, and after the final END, and must not mistake dashes inside that text for a
boundary. Exercised through both the low-level TPemReader and the OpenSSL-level
TOpenSslPemReader. Input lives in a new fixture (Data/Pem/Reader/ExplanatoryText.txt)
loaded via TPemReaderVectors, matching the existing PEM tests.