rsa: add missing exponent too small check

[MichaelMure]

Note

Low Risk
Tightens input validation on RSA key import with new tests; only rejects previously accepted invalid exponents like 1.

Overview
PublicKeyFromNE now rejects public exponents < 2 (e.g. exponent 1) with exponent too small, in addition to existing positive/odd/int64 checks. That path is used when building keys from raw N/E bytes and from PrivateKeyFromNEDPQ.

key_test.go adds round-trip tests for PKCS#1 DER, N/E, and N/E/D/P/Q reconstruction, plus TestRejectWeirdPublicKeyInputs table tests that assert PublicKeyFromNE errors on bad moduli (empty, short, oversized, even) and bad exponents (empty, one, even, too large for int64).

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