Fix misleading key_available in inno comply --encryption-audit

[ringo380]

Summary

inno comply --encryption-audit derived key_available from encryption_info().is_some(), which is exactly what is_encrypted() returns - both read the same page-0 encryption info. So the report showed key available: yes for any encrypted tablespace even when no --keyring was passed, giving the opposite of a useful signal in a forensic/compliance context.

Fix

• Add Tablespace::has_decryption_key(), which returns whether a decryption context was actually installed via set_decryption_context() (i.e. a usable key is present).
• Use it for key_available in encryption_audit().

Now an encrypted tablespace reports key available: no until a working --keyring is supplied, and yes once decryption is set up. Plaintext behavior is unchanged (still no).

Testing

• cargo fmt --check, release build, and clippy clean on the changed files.
• Existing compliance/comply tests pass (13 lib + 17 integration).
• --encryption-audit on the plaintext fixture reports Key available: no as expected.

This was surfaced during review of #211 and left as a follow-up because it needed a new accessor on tablespace.rs.