fix: repair invalid UTF-8 agent metadata cache fields

[piorpua]

Belongs to AionIssue fix run 128150775/001.

Related repositories in this fix run: AionCore, AionUi, aionrs.

Summary:

• Read rebuildable agent_metadata cache fields as bytes and clear invalid UTF-8 values to NULL without dropping agent rows.
• Route list, get, source/name, builtin backend, apply_handshake, and agent binding reads through the safe agent metadata mapper.
• Keep registry hydration and AppServices health startup moving when old cache data is corrupted.
• Preserve valid UTF-8 invalid JSON behavior on the existing registry fallback path.

Validation:

• cargo test -p aionui-db invalid_utf8 -- --nocapture
• cargo test -p aionui-db resolve_agent_binding_uses_safe_agent_metadata_reads -- --nocapture
• cargo test -p aionui-ai-agent hydrate_continues_when_agent_metadata_config_options_has_invalid_utf8 -- --nocapture
• cargo test -p aionui-ai-agent hydrate_keeps_valid_utf8_invalid_json_config_options_non_fatal -- --nocapture
• cargo test -p aionui-ai-agent registry_config_option -- --nocapture
• cargo test -p aionui-app health_check_returns_ok_when_agent_metadata_cache_field_has_invalid_utf8 -- --nocapture
• cargo fmt --all --check
• cargo test -p aionui-db
• cargo test -p aionui-ai-agent
• cargo clippy -p aionui-db -p aionui-ai-agent --all-targets -- -D warnings
• rg -n 'aionrs.git", tag = "v0.1.33"' Cargo.toml

Notes:

• No SQLite schema migration is added.
• aionrs remains a validation-only related repository for this fix run.