revert: S26.04 mbedTLS client-side TLS session resumption

[DavidCozens]

Reverts S26.04 (#489, commit d37ef31). TLS session resumption is now out of scope for both reference TLS integrations (Platform/OpenSsl/, Platform/MbedTls/), on either backend, for either TLS 1.2 or TLS 1.3.

The S26.04 production diff was purely additive (42 add / 0 del) with no bundled bug fix to preserve, so the git revert is total and clean (single-parent squash). A new DEVLOG entry records the decision and rationale.

Why

1. Resumption is an optimization, never correctness — a full handshake must always succeed on every (re)connect regardless.
2. Long-lived keepalive connections make it near-worthless here — one handshake, then a long-lived stream; resumption only pays at reconnect, and reconnects are rare.
3. When reconnects do happen, resumption is least likely to be available — a reconnect is usually a disgraceful close, exactly when the server has discarded resumable state.
4. Disproportionate cost to do honestly — TLS 1.2 (RFC 5077) vs 1.3 (RFC 8446 PSK) differ; 1.3 tickets arrive post-handshake; mTLS-on-resume has separate identity semantics. Four cases × honest mechanism-correct tests for an optional speedup.

Companion work: OpenSSL S26.01 (#280) closed won't-do; the SSL_CTX-on-Close leak found alongside it lands separately in #491.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

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Run ID: 70d1e551-4ec1-431a-8b0e-55c8923fb61a

📥 Commits

Reviewing files that changed from the base of the PR and between d37ef31 and f919179.

📒 Files selected for processing (14)

• DEVLOG.md
• Platform/MbedTls/Source/SolidSyslogMbedTlsStream.c
• Platform/MbedTls/Source/SolidSyslogMbedTlsStreamPrivate.h
• Tests/MbedTls/SolidSyslogMbedTlsStreamTest.cpp
• Tests/MbedTlsIntegration/CMakeLists.txt
• Tests/MbedTlsIntegration/MbedTlsResumptionServer.c
• Tests/MbedTlsIntegration/MbedTlsResumptionServer.h
• Tests/MbedTlsIntegration/ReconnectingSocketStream.c
• Tests/MbedTlsIntegration/ReconnectingSocketStream.h
• Tests/MbedTlsIntegration/SolidSyslogMbedTlsStreamIntegrationTest.cpp
• Tests/Support/MbedTlsFake.c
• Tests/Support/MbedTlsFake.h
• docs/integrating-mbedtls.md
• misra_suppressions.txt

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch revert/mbedtls-session-resumption

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[github-actions]

☀️   Quality Summary

   🚦   build-linux-gcc: 100% successful (✔️ 1344 passed)
   🚦   build-freertos-host-tdd-plustcp: 100% successful (✔️ 1652 passed)
   🚦   build-linux-clang: 100% successful (✔️ 1296 passed)
   🚦   sanitize-linux-gcc: 100% successful (✔️ 1296 passed)
   🚦   integration-linux-openssl: 100% successful (✔️ 9 passed)
   🚦   integration-linux-mbedtls: 100% successful (✔️ 7 passed)
   🚦   integration-windows-openssl: 100% successful (✔️ 9 passed)
   🚦   bdd-linux-syslog-ng: 94% successful (✔️ 46 passed, 🙈 3 skipped)
   🚦   bdd-windows-otel: 90% successful (✔️ 44 passed, 🙈 5 skipped)
   🚦   bdd-freertos-qemu-plustcp: 86% successful (✔️ 42 passed, 🙈 7 skipped)
   🚦   bdd-freertos-qemu-lwip: 65% successful (✔️ 32 passed, 🙈 17 skipped)
   🚦   build-windows-msvc: 100% successful (✔️ 1149 passed)
   🚦   build-linux-tunable-override: 100% successful (✔️ 1296 passed)
   ⚠️   Clang-Tidy: No warnings
   ⚠️   CPPCheck: No warnings

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