Add Mbed TLS 4.x support (PSA Crypto) (#2502)

* Add Mbed TLS 4.x support (PSA Crypto) for macOS

Auto-detect Mbed TLS 4.x via MBEDTLS_VERSION_MAJOR and adapt the backend:
- Include psa/crypto.h and drop the headers removed in 4.x (ctr_drbg,
  entropy, md5, sha*), gated behind the version macro.
- Compute MD5/SHA-256/SHA-512 via PSA (psa_hash_compute) and initialize
  PSA Crypto once with std::call_once.
- Drop the explicit entropy/CTR-DRBG RNG (PSA provides the TLS RNG) and
  skip the RNG-callback overloads of pk_parse_key/pk_check_pair on 4.x.
- Retry on a TLS 1.3 NewSessionTicket (the 4.x default) in connect, read,
  write and is_peer_closed via a single mbedtls_is_session_ticket() helper,
  so online HTTPS works, including large redirected downloads where the
  ticket arrives mid-write.

Note V4 implies V3, so 3.x-only paths now check V3 && !V4.

Build systems (macOS): the CMake config and pkg-config shipped by Homebrew
resolve 4.x transitively, so CMakeLists.txt and meson.build need no change
for linking; the Makefile links libtfpsacrypto when present, else
libmbedcrypto.

Tests: generate the encrypted client key as both PBES2-AES (3.6+/4.x,
OpenSSL, wolfSSL) and PBES1-3DES (Mbed TLS 2.28) and pick by version, since
4.x dropped DES and 2.28 lacks PBES2. Also generate the IP-host certs in
test/meson.build to match gen-certs.sh and CMakeLists.txt.

* CI: test Mbed TLS 4.x on macOS, 3.x on Ubuntu 26.04

Homebrew's default mbedtls is now 4.x, so switch the macOS build and CI job
to it (drop the mbedtls@3 pin). That leaves 3.x (Ubuntu 24.04 apt ships 2.28,
macOS now 4.x) uncovered, so add an ubuntu-26.04 job whose apt provides Mbed
TLS 3.6. Net coverage: 2.28 (ubuntu-latest), 3.6 (ubuntu-26.04), 4.2 (macOS).

ubuntu-26.04 is a public-preview runner image; fold it into the main ubuntu
matrix once ubuntu-latest moves to 26.04.

* Document Mbed TLS 4.x support and libtfpsacrypto rename

Update README.md and the tour's TLS setup pages (en/ja) to note that
Mbed TLS 4.x is now auto-detected and that it renames libmbedcrypto
to libtfpsacrypto.

docs-src/pages/en/tour/05-tls-setup.md

@@ -79,6 +79,8 @@ cpp-httplib also supports Mbed TLS and wolfSSL in addition to OpenSSL. You can s
 | Mbed TLS | `CPPHTTPLIB_MBEDTLS_SUPPORT` | `libmbedtls`, `libmbedx509`, `libmbedcrypto` |
 | wolfSSL | `CPPHTTPLIB_WOLFSSL_SUPPORT` | `libwolfssl` |
 
+Mbed TLS 2.x, 3.x, and 4.x are all supported and auto-detected. Note that Mbed TLS 4.x renames `libmbedcrypto` to `libtfpsacrypto`, so link against that instead.
+
 This tour assumes OpenSSL, but the API is the same regardless of which backend you choose.
 
 ## Next Step

docs-src/pages/ja/tour/05-tls-setup.md

@@ -79,6 +79,8 @@ cpp-httplibはOpenSSL以外にも、Mbed TLSとwolfSSLに対応しています
 | Mbed TLS | `CPPHTTPLIB_MBEDTLS_SUPPORT` | `libmbedtls`, `libmbedx509`, `libmbedcrypto` |
 | wolfSSL | `CPPHTTPLIB_WOLFSSL_SUPPORT` | `libwolfssl` |
 
+Mbed TLSは2.x、3.x、4.xいずれも対応していて、自動判定されます。4.xでは`libmbedcrypto`が`libtfpsacrypto`という名前に変わっているので、リンクするライブラリはそちらに読み替えてください。
+
 このTourではOpenSSLを前提に進めますが、APIはどのバックエンドでも共通です。
 
 ## 次のステップ