test(java): add RSADigestSigner constructor edge-case coverage

[sachin9058]

This PR extends regression coverage for BouncyCastle signer edge cases related to RSADigestSigner.

What changed

• Added a fixture covering the digest-only RSADigestSigner constructor path
• Extended the existing test expectations to validate the additional finding
• Verified the generated CBOM output remains consistent with the current RSA signature and digest relationship mapping

Investigation Notes

While working on this issue, I also reviewed the existing EC/DSA signer paths:

• ECDSASigner support is already implemented through the existing BcDSA rule and mapper
• Existing test coverage for that path is already present
• DSADigestSigner currently resolves through the generic DSA translation flow, so I intentionally avoided adding misleading EC-specific assertions

This keeps the PR scoped specifically to improving regression coverage without changing current translation semantics.

Validation

mvn -pl java -Dtest=BcRSADigestSignerTest test
mvn test -pl java

Related

Closes part of #31

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[san-zrl]

Hi @sachin9058 - LGTM. Thank you for your contribution to CBOMkit. This is highly appreciated.