Skip TCP MTU checks when TCP is disabled

[HTRamsey]

Description

When TCP is disabled, vPreCheckConfigs() still asserted that the configured MTU could hold a TCP header and ipconfigTCP_MSS. FreeRTOSIPConfigDefaults.h also enforced the TCP MSS minimum and overflow checks
unconditionally. These TCP-only constraints could reject otherwise valid UDP-only configurations.

This change:

• Moves the MTU/MSS assertion into the existing ipconfigUSE_TCP guard.
• Applies the MSS minimum and overflow validation only when TCP is enabled.
• Changes the DISABLE_ALL build configuration to use the minimum valid 46-byte MTU, providing compile coverage for TCP-disabled, small-MTU configurations.

TCP-enabled configurations retain the existing MSS and MTU protections.

Test Steps

• Built the complete freertos_plus_tcp core library with FREERTOS_PLUS_TCP_TEST_CONFIGURATION=DISABLE_ALL and ipconfigNETWORK_MTU=46.
• Built the complete freertos_plus_tcp core library with FREERTOS_PLUS_TCP_TEST_CONFIGURATION=ENABLE_ALL_IPV4_TCP.
• Confirmed that a TCP-enabled syntax check with ipconfigTCP_MSS=535 still fails with the expected minimum-MSS configuration error.
• Ran git diff --check.

Checklist:

• I have tested my changes. No regression in existing tests.
• [] I have modified and/or added unit-tests to cover the code changes in this Pull Request.

The DISABLE_ALL build-combination configuration was updated to cover the TCP-disabled, minimum-MTU case.

Related Issue

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[htibosch]

Thank you @HTRamsey for this PR. I approve it.