THRIFT-5759: Remove mbstring.func_overload workaround from PHP library

[sveneld]

Removes the TStringFunc abstraction whose only purpose was to work around mbstring.func_overload, a PHP ini setting that was deprecated in PHP 7.2 and removed in PHP 8.0. Since THRIFT-5956 raised the project's minimum PHP version to 8.1, the workaround has been functionally unreachable on every supported runtime — ini_get('mbstring.func_overload') always returns false. This PR rips out the dead code.

This is not a behavior change on PHP 8.1+: TStringFuncFactory::create()->strlen($x) and native strlen($x) already return the same byte count, which is what Thrift's binary protocols require. The Thrift code generator does not emit any TStringFunc* references, so generated user code is unaffected.

What changed

• Deleted Thrift\Factory\TStringFuncFactory, Thrift\StringFunc\{Core, Mbstring, TStringFunc}, and the legacy non-namespaced lib/php/src/TStringUtils.php.
• Replaced 30+ TStringFuncFactory::create()->strlen/substr call sites in lib/Transport/* and lib/Protocol/* with native strlen() / substr().
• Initialized TFramedTransport::$rBuf_ and $wBuf_ to '' (instead of the implicit null default) and reset $rBuf_ to '' after a frame is consumed, so the new direct strlen()/substr() calls never receive null and never trigger the PHP 8.1 "passing null to parameter of type string is deprecated" warning that the deleted Core wrapper used to mask via (string)$str casts.
• Removed the now-meaningless unit tests (TStringFuncFactoryTest, CoreTest, MbStringTest) and the obsolete mbstring.func_overload=0 ini override in src/ext/thrift_protocol/run-tests.php.
• Added a regression test StringByteCountTest that pins the byte-count contract: TBinaryProtocol/TCompactProtocol round-trip non-ASCII UTF-8 payloads (Cyrillic, Arabic, CJK, Malayalam, emoji, etc.) intact, and the on-the-wire i32 length prefix matches the byte length, not the character count. The test also covers TFramedTransport over TMemoryBuffer to exercise the rBuf_ exhaustion path.
• No mb_* calls remain anywhere in lib/php, test/php, the tutorial, or the C extension — mbstring is therefore dropped from build.yml, sca.yml, and the focal/jammy/noble Dockerfiles.
• Updated lib/php/Makefile.am to drop the phpstringfuncdir block and the src/TStringUtils.php distribution entry; src/Thrift.php no longer includes the deleted file.

Diff: 27 files changed, +171 / −873.

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• Did you create an Apache Jira ticket? — THRIFT-5759
• If a ticket exists: Does your pull request title follow the pattern "THRIFT-NNNN: describe my issue"? — yes
• Did you squash your changes to a single commit? — single commit d25e8eaa
• Did you do your best to avoid breaking changes? If one was needed, did you label the Jira ticket with "Breaking-Change"? — no breaking change. The removed classes (Thrift\Factory\TStringFuncFactory, Thrift\StringFunc\*) only existed to detect a PHP feature that no longer exists; on every supported runtime they were no-op wrappers around native strlen()/substr(). Behavior is identical for any caller on PHP 8.1+.
• If your change does not involve any code, include [skip ci] anywhere in the commit message to free up build resources. — N/A, this PR is code.