codegen: add with_* builder-style setter methods for explicit-presence fields (#30)

[tejas-dharani]

Closes #30.

Setting explicit-presence fields during construction required mutating a default() in place — no chaining, no Some(...) elision.

Problem

let mut req = GetSecretRequest::default();
req.name = Some("alice".to_string());
req.timeout_ms = Some(30_000);
req.enabled = Some(true);

Fix

Every explicit-presence scalar, bytes, and enum field now gets a with_<name> setter:

let req = GetSecretRequest::default()
    .with_name("alice")
    .with_timeout_ms(30_000)
    .with_enabled(true);

The setter signature is pub fn with_<name>(mut self, value: impl Into<T>) -> Self, decorated with #[must_use] and #[inline]. impl Into<T> is used where a natural conversion exists: String accepts &str; Vec<u8> bytes accepts b"..." literals (From<&[T; N]> for Vec<T> stable since Rust 1.74, MSRV is 1.85); bytes::Bytes accepts Vec<u8>; open-enum fields accept the bare variant directly — EnumValue<E>: From<E> means with_priority(Priority::HIGH) works without EnumValue::Known(...). Plain scalars take the bare type.

The gate is inner_opt_type.is_some() — the bare inner T from classify_field — rather than is_optional. Proto2 repeated fields have is_optional = true (proto2's EXPLICIT-presence default) while their struct field is Vec<T>, not Option<T>; the gate correctly excludes them. Message fields, repeated, map, oneof variants, and proto2 required fields are all excluded for the same reason. To clear a field: msg.name = None;.

Opt out per-compilation unit with CodeGenConfig { generate_with_setters: false, ..Default::default() }.

Tests

Six integration tests cover: setters present for explicit-presence fields and absent for implicit/repeated; generate_with_setters = false suppresses all setters; Vec<u8>, bytes::Bytes, and enum setters each use impl Into; proto2 repeated produces no setter (regression guard for the gate).

Seven runtime tests cover: chained construction, overwrite semantics, unset fields stay None, encode/decode round-trip, &str for strings, b"..." for bytes, and bare variant for enums.

Semver

Additive — new inherent methods on all generated message types.

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