[fix](cloud) Validate server config before starting services

[Yukang-Lian]

Proposed changes

• Validate Cloud server configuration before initializing TxnKv, MetaService, Recycler, or background threads.
• Keep start-time validation as a defensive check.
• Add OSS starter unit coverage for plaintext, unsupported TLS, and null-server configurations.

Problem

Cloud server validation currently runs after MetaService, Recycler, and the Recycler logging thread have started. If validation rejects the configuration, main returns through partially initialized resources, and the joinable Recycler thread can terminate the process during stack unwinding.

[hello-stephen]

Thank you for your contribution to Apache Doris.
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Please clearly describe your PR:

1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
2. Which behaviors were modified. What was the previous behavior, what is it now, why was it modified, and what possible impacts might there be.
3. What features were added. Why was this function added?
4. Which code was refactored and why was this part of the code refactored?
5. Which functions were optimized and what is the difference before and after the optimization?

[Yukang-Lian]

run buildall

[github-actions]

PR approved by at least one committer and no changes requested.

[github-actions]

PR approved by anyone and no changes requested.

[hello-stephen]

Cloud UT Coverage Report

Increment line coverage 42.86% (3/7) 🎉

Increment coverage report
Complete coverage report

Category	Coverage
Function Coverage	77.54% (2023/2609)
Line Coverage	64.96% (36866/56748)
Region Coverage	65.48% (18732/28609)
Branch Coverage	55.11% (10203/18514)