Add input length validation for agent fields

[mduongvandinh]

Summary

• Add length limits to agent name, description, and system prompt fields
• Prevents unbounded string storage in SQLite

Changes

• MAX_AGENT_NAME_LEN (256 chars) for name in patch_agent_config and new_name in clone_agent
• MAX_AGENT_DESCRIPTION_LEN (4096 chars) for description in patch_agent_config
• MAX_SYSTEM_PROMPT_LEN (64KB) for system_prompt in patch_agent_config
• Returns 413 Payload Too Large with descriptive error message

Test plan

• cargo build -p openfang-api --lib passes
• cargo clippy -p openfang-api --all-targets -- -D warnings zero warnings
• cargo fmt --all --check clean
• Follows existing MAX_MESSAGE_SIZE validation pattern

Files changed

• crates/openfang-api/src/routes.rs (+43)