Keep wire-generated Kotlin clean under extra compiler checks

[oldergod]

Kotlin 2.1 added extra compiler checks (-Wextra). Two of them flag wire-generated code. Users cannot fix generated code, so the fix belongs in wire.

Fixes #3700
Fixes #3701

Changes

hashCode() for mutable types (#3701, VARIABLE_INITIALIZER_IS_REDUNDANT). The generator emitted var result = 0, and the next line overwrote the value with unknownFields.hashCode(). The generator now initializes the variable with unknownFields.hashCode() directly. The immutable path is unchanged: its super.hashCode initializer is read by the cache check.

encodedSize() for messages with no fields (#3700, CAN_BE_VAL_DELAYED_INITIALIZATION). The body degenerated to var size = value.unknownFields.size; return size. The generator now emits return value.unknownFields.size with no local variable. KotlinPoet renders this as an expression body.

Regression gate. wire-golden-files now compiles with extraWarnings enabled. Four diagnostics fail the build via -Xwarning-level=<NAME>:error: VARIABLE_INITIALIZER_IS_REDUNDANT, CAN_BE_VAL_DELAYED_INITIALIZATION, CAN_BE_VAL, and CAN_BE_VAL_LATEINIT. Plain allWarningsAsErrors is too broad here: it also fails on repo-wide compiler warnings that are unrelated to the goldens, for example the deprecated -Xjvm-default flag spelling and the deprecated language version 2.0. REDUNDANT_VISIBILITY_MODIFIER is disabled for this module: KotlinPoet emits explicit visibility modifiers on purpose, and consumers with explicit API mode need them. Without this, it fires 984 times on the goldens.

Verification

• Gate discrimination: with the gate and the old generator, :wire-golden-files:compileKotlin fails with exactly the four expected sites (three in hashCode, one in encodedSize). With the fixed generator, the build passes.
• The compiler rejects unknown diagnostic names with an error, so a green build proves the four escalated names are valid.
• :wire-kotlin-generator:test: 75 tests, 0 failures, forced fresh with --rerun-tasks --no-build-cache.
• spotlessCheck and apiCheck pass. The golden files change shape only. There is no runtime or .api change.
• All checked-in generated Kotlin lives in wire-golden-files. A repo-wide grep found no other fixture with the old shapes.