rsa: enforce key size limits on all constructors (priv+pub)

[MichaelMure]

Note

Medium Risk
Tightens cryptographic key acceptance on all import paths; behavior change for callers that relied on sub-2048-bit or malformed keys, but it hardens security rather than widening attack surface.

Overview
Centralizes RSA public-key policy in a new validatePublicKey helper (modulus bit length within MinRsaKeyBits/MaxRsaKeyBits, odd modulus, valid exponent) and runs it on every public import path that previously only parsed DER/PEM or NE bytes—PKCS#1, PKIX/X509, multibase (via X509), and PublicKeyFromNE (inline checks replaced by the shared validator).

Private PKCS#8 imports now reject weak keys by validating the embedded public half, then call rsa.PrivateKey.Validate() and Precompute() before wrapping the key.

Adds TestRejectBelowPolicyRSAImports to assert 1024-bit material fails across generation and all listed public/private constructors.

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[Copilot]

Pull request overview

This PR hardens RSA key handling by enforcing a consistent public-key policy (minimum/maximum modulus size plus basic modulus/exponent validity) across all RSA import/constructor paths, and strengthens private key imports by validating and precomputing the parsed RSA private key material.

Changes:

• Centralizes RSA public-key validation in a new validatePublicKey helper and applies it to PKCS#1, X.509 (DER/PEM), multibase (via X.509), and PublicKeyFromNE.
• Tightens PKCS#8 private key imports by validating the embedded public key and calling rsa.PrivateKey.Validate() + Precompute() before returning.
• Adds a regression test asserting 1024-bit RSA material is rejected across generation and all relevant public/private constructors.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
crypto/rsa/public.go	Adds shared RSA public-key validation and applies it to all public-key constructors.
crypto/rsa/private.go	Enforces policy and strengthens PKCS#8 private key import by validating and precomputing parsed keys.
crypto/rsa/key_test.go	Adds coverage ensuring sub-policy RSA keys are rejected across all constructors/import paths.

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