ffi: add security rule enumeration API

[ronaldtse]

Summary

Adds two FFI functions that let callers enumerate the security profile's rule list:

• rnp_get_security_rule_count() — total number of rules (defaults + user-added)
• rnp_get_security_rule_at() — fetch one rule by index with type/name/level/from/flags fields

Caller owns type and name strings; free via rnp_buffer_destroy.

Why

Today the only way to inspect the active security profile is rnp_get_security_rule(), which requires you to already know the feature type and name. There is no way to enumerate the built-in defaults (SHA-1, MD5, CAST5, 3DES, IDEA, BLOWFISH, RIPEMD) or iterate the full rule list.

This matters for:

• Downstream tooling (packagers, distros, security auditors) that wants to report which algorithms rnp considers insecure on a given date.
• Diagnostics — e.g. "what does this rnp build consider weak right now?" without needing to query feature-by-feature.
• Library consumers building key-management UIs that surface algorithm deprecation.

Implementation

• src/lib/sec_profile.{hpp,cpp}: add SecurityProfile::rules() returning a const ref to the internal vector.
• src/lib/rnp.cpp: add the two FFI functions. They reuse the existing hash_alg_map / symm_alg_map / pubkey_alg_map tables for feature int → name lookup, the existing ret_str_value() helper for string allocation, and the existing RNP_SECURITY_* constants for output.
• src/tests/ffi.cpp: new test_ffi_security_rule_enumeration covering NULL checks, default count (3 hashes + 4 ciphers, +RIPEMD under crypto-refresh), out-of-range index, enumeration of MD5/CAST5/SHA1-data/SHA1-key rules, NULL output parameters, and that adding a rule is reflected in the count.

Test plan

• rnp_tests --gtest_filter=rnp_tests.test_ffi_security_rule_enumeration passes locally
• rnp_tests --gtest_filter=rnp_tests.test_ffi_security_profile still passes (no regression)
• Build clean against main with crypto refresh enabled
• CI green on all platforms

[codecov]

Codecov Report

❌ Patch coverage is 90.90909% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.38%. Comparing base (953d2e0) to head (6be024f).

Files with missing lines	Patch %	Lines
src/lib/rnp.cpp	90.62%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2424      +/-   ##
==========================================
+ Coverage   85.36%   85.38%   +0.01%     
==========================================
  Files         126      126              
  Lines       22861    22932      +71     
==========================================
+ Hits        19516    19580      +64     
- Misses       3345     3352       +7     

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[ni4]

LGTM!