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.

test/test.cc

@@ -47,7 +47,13 @@ inline std::string u8_to_string(const char8_t *s) {
 #define CLIENT_CERT_FILE "./client.cert.pem"
 #define CLIENT_PRIVATE_KEY_FILE "./client.key.pem"
 #define CLIENT_ENCRYPTED_CERT_FILE "./client_encrypted.cert.pem"
+// Mbed TLS < 3.6 (e.g. Ubuntu's 2.28) has no PBES2-AES and needs the PBES1-3DES
+// key; 3.6+/4.x (4.x dropped DES) and OpenSSL/wolfSSL use the PBES2 AES key.
+#if defined(CPPHTTPLIB_MBEDTLS_SUPPORT) && (MBEDTLS_VERSION_NUMBER < 0x03060000)
+#define CLIENT_ENCRYPTED_PRIVATE_KEY_FILE "./client_encrypted_pbes1.key.pem"
+#else
 #define CLIENT_ENCRYPTED_PRIVATE_KEY_FILE "./client_encrypted.key.pem"
+#endif
 #define CLIENT_ENCRYPTED_PRIVATE_KEY_PASS "test012!"
 #define SERVER_ENCRYPTED_CERT_FILE "./cert_encrypted.pem"
 #define SERVER_ENCRYPTED_PRIVATE_KEY_FILE "./key_encrypted.pem"
@@ -18505,9 +18511,10 @@ TEST(SSLClientServerTest, CustomizeServerSSLCtxMbedTLS) {
       if (mbedtls_x509_crt_parse_file(&own_cert, SERVER_CERT_FILE) != 0) {
         return false;
       }
-      // Load server private key
+      // Load server private key.
+      // Mbed TLS 3.x takes an RNG callback here; 2.x and 4.x do not.
       if (mbedtls_pk_parse_keyfile(&own_key, SERVER_PRIVATE_KEY_FILE, nullptr
-#if MBEDTLS_VERSION_MAJOR >= 3
+#if MBEDTLS_VERSION_MAJOR == 3
                                    ,
                                    mbedtls_ctr_drbg_random, nullptr
 #endif