Harden PHP repeated field indexing and namespace option validation

[Abhinav-143x]

This PR adds defensive validation to two PHP code paths where malformed input can otherwise reach native runtime or generated-code boundaries.

First, the native PHP RepeatedField::offsetSet() path now explicitly rejects negative and out-of-range indexes before converting the assigned value and before casting the index for upb_Array_Set(). This makes offsetSet() consistent with the other ArrayAccess methods and prevents invalid user-provided indexes from reaching the native repeated-field setter path.

Second, the PHP generator now validates php_namespace and php_metadata_namespace before using those options to derive PHP namespaces and generated filenames. This prevents malformed schema options from being emitted into generated PHP namespace/file output. Existing valid forms remain accepted, including empty options, normal namespace segments such as Php\\Test, and the existing php_metadata_namespace = "\\" root metadata special case.

Tests run:

• bazel test //src/google/protobuf/compiler/php:generator_unittest
• bazel build //php:extension
• bazel test //php:conformance_test_c
• bazel test //php:conformance_test
• bazel test //php:proto_staleness_test
• bazel test //php:test_amalgamation_staleness