docs(serde): document and test supported serde patterns for IPv4 / IPv6 / FixedString

[gregvirgin-ls]

Summary

This is not a bug fix — the wire format is already correct. What was missing was:

• An explicit rustdoc statement of the required-annotation contract for Ipv4Addr (clickhouse::serde::ipv4 is required because the default Ipv4Addr serde produces 4 network-order bytes whereas the IPv4 wire format is little-endian u32).
• A symmetric helper for Ipv6Addr (clickhouse::serde::ipv6) — pass-through, but available so mixed-family row definitions read consistently.
• Byte-level unit tests pinning the round-trip wire format for all three shapes.
• A live integration test that exercises the documented annotations against a real ClickHouse server with schema validation enabled.

What changed

1. src/serde.rs: rustdoc on clickhouse::serde::ipv4 made explicit; new clickhouse::serde::ipv6 pass-through helper.
2. src/rowbinary/tests.rs: byte-level round-trip tests under mod ip_and_fixed_string covering IPv4, IPv6 (default and helper), and FixedString(4).
3. tests/it/ip.rs: new ipv4_ipv6_fixedstring_round_trip test creating a table with all three column types, inserting via #[derive(Row, Serialize)], fetching with validation enabled, and asserting both Rust-side equality and CH-side toString(...) equivalence.

What is NOT in this PR

• A PaddedString<N> (or similar) newtype that would let users write String and have it auto-pad-or-truncate to FixedString(N). Out of scope; if maintainers want it, follow-up.
• Any change to existing Ipv4Addr / Ipv6Addr serde behaviour. The contract is unchanged; it's now documented.

Behaviour change

None observable to existing code. The new clickhouse::serde::ipv6 helper is opt-in. Existing users who rely on the default Ipv6Addr serde keep working.

Test matrix

cargo fmt -- --check
cargo clippy --all-targets --no-default-features
cargo clippy --all-targets --all-features
cargo clippy --features rustls-tls
cargo test --workspace --no-default-features
cargo test --workspace
cargo test --workspace --all-features
cargo rustdoc -p clickhouse --all-features -- -D warnings --cfg docsrs

The live IT test (tests/it/ip.rs::ipv4_ipv6_fixedstring_round_trip) requires a running ClickHouse instance, per the existing convention for tests/it/.

Downstream motivation

A non-trivial fraction of clickhouse-rs consumers we've looked at use String columns to hold IPs because the round-trip via Ipv4Addr / Ipv6Addr was opaque enough to be distrusted in production. Putting the contract in rustdoc plus pinning it with a live IT test resolves the discoverability question.