fix(libtls): enable SECP384R1 and raise default TLS content length fo…

[yingtao450]

…r US region

US-region cloud chains (rtc-ai*.iot-wus.com -> DigiCert Global Root G3) carry an ECDSA P-384 root certificate. With only SECP256R1 compiled in, certificate parsing fails with MBEDTLS_ERR_PK_UNKNOWN_NAMED_CURVE (-0x3a00) during the TLS handshake, so US-region AI connections never establish while CN region (RSA/P-256 chains) works fine.

Also raise ENABLE_MBEDTLS_SSL_MAX_CONTENT_LEN default from 1024 to 4096: the US chain Certificate message is ~2.6KB (leaf 1187 + intermediate 844

• root 579), which exceeds 1024 unless the peer honors max_fragment_length.

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在本次拉取请求中，我已考虑以下事项 As part of this pull request, I've considered the following:

• 确保代码注释和文档清晰，并使用英文注释以保证代码可读性。Ensure that the code comments and documentation are clear, and use English for comments to ensure code readability.
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• 已使用代码格式化工具确保符合 TuyaOpen 编码规范。Have used the code-formatting source code formatting tool to ensure compliance with TuyaOpen coding standards.