THRIFT-5990: Emit native return types on generated PHP struct methods

[sveneld]

Phase G2 of the THRIFT-5960 PHP modernization. The PHP code generator now emits native return types on the public struct surface.

Generated method signatures:

Method	Standard	Binary-inline mode
getName()	: string	: string
read()	(TProtocol $input): int	($input): int (1)
write()	(TProtocol $output): int	(string &$output): int
validateForRead/Write()	: void	: void
jsonSerialize()	: mixed (2)	n/a

(1) Binary-inline mode reads from a raw TTransport buffer, not a TProtocol, so the param remains untyped to avoid pulling another import into every generated file in this PR. (Can be typed in a follow-up.)
(2) The previously-emitted #[\ReturnTypeWillChange] attribute is dropped since the project targets PHP 8.1+, which natively supports the JsonSerializable::jsonSerialize(): mixed interface.

Library change: tighten TBase abstracts in lock-step.

To make TProtocol $input / TProtocol $output in the generated subclass methods compatible with PHP's LSP rules, the abstract methods on Thrift\Base\TBase are tightened:

- abstract public function read($input);
- abstract public function write($output);
+ abstract public function read(TProtocol $input): int;
+ abstract public function write(TProtocol $output): int;

Any third-party generated PHP code from an older Thrift compiler that extends TBase will need to be regenerated against this compiler.

Hand-written test fixtures updated:

• lib/php/test/Unit/Lib/Fixture/TestSerializerStruct.php
• lib/php/test/Unit/Lib/Base/Fixture/ComplexStruct.php
• lib/php/test/Unit/Lib/Base/Fixture/NestedStruct.php

These extend TBase and were updated to match the new abstract signatures.

Generator fixtures regeneration:

lib/php/test/Resources/packages/ is git-ignored and is regenerated by make stubs in CI before tests run. No fixture files appear in this diff.

Validation (Docker thrift/thrift-build:ubuntu-focal + thrift-php-dev:local):

• Built compiler with the patched generator.
• Regenerated all 6 fixture variants (435 PHP files).
• phpcs — 0 errors.
• phpstan (level 1) — 0 errors.
• phpunit Unit Suite — 635 tests, 0 failures.
• phpunit Integration Suite — 108 tests, 0 failures.

Scope:

This is phase G2. The remaining phase G3 will emit native types on generated property declarations and the __construct(?array $vals = null) parameter, completing the generator side of THRIFT-5960.

• Did you create an Apache Jira ticket? — THRIFT-5990
• PR title follows the pattern "THRIFT-NNNN: describe my issue"
• Squashed to a single commit
• Did you do your best to avoid breaking changes? — TBase abstract signature change is a deliberate, scoped break that affects only direct extenders of TBase. In practice that's only Thrift-generated code, which is regenerated.

Generated-by: Claude Opus 4.7