checker: support TSO microservice config in active active preflight

[King-Dylan]

What problem does this PR solve?

Active-active TSO preflight still reads tso-unique-index and tso-max-index from PD config only. In TSO microservice deployments, that can validate the wrong source of truth and reject or accept changefeed creation incorrectly.

Issue Number: close #5091

What is changed and how it works?

• read downstream TSO indexes from SHOW CONFIG with type='tso' first and fall back to type='pd' only when no TSO rows are reported
• read upstream TSO indexes from the TSO microservice config endpoint when the microservice API is available, and fall back to PD config only for legacy deployments
• keep the validation fail-closed for missing configs, empty TSO membership, inconsistent instance values, and TSO config read failures
• reuse the HTTP client when reading multiple TSO member configs and keep bounded request timeouts

Check List

Tests

• Unit test

Questions

Will it cause performance regression or break compatibility?

No. The change keeps the old PD fallback for legacy embedded-TSO deployments, and it reuses the HTTP client when reading multiple TSO member configs.

Do you need to update user documentation, design documentation or monitoring documentation?

No.

Release note

TiCDC active-active TSO preflight now reads TSO microservice config when it is available and falls back to PD config for legacy deployments.

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All committers have signed the CLA.

[gemini-code-assist]

Code Review

This pull request introduces support for TSO microservices in the active-active TSO index validation logic. It updates both upstream and downstream retrieval processes to prioritize TSO service configurations while maintaining fallbacks to legacy PD configurations. Feedback focuses on optimizing the retrieval of upstream TSO configurations by reusing HTTP clients to avoid connection overhead and improving code clarity by explicitly returning nil in success paths.

[gemini-code-assist]

Code Review

This pull request adds support for TSO microservices to the active-active TSO index validation logic, updating both upstream and downstream retrieval to prioritize TSO service configurations with a fallback to legacy PD settings. The review feedback identifies a performance optimization to reuse HTTP clients rather than recreating them in loops and suggests a minor refactor to return nil explicitly when no error is present.