Support creating changefeeds in a paused state

Is your feature request related to a problem?

Creating a changefeed currently starts replication immediately. Some workflows need to validate and save the changefeed and protect its starting position, then explicitly start replication later.

Describe the feature you'd like

Support an optional pause parameter on POST /api/v2/changefeeds and top-level pause = true in the CLI TOML configuration.

• Omitted or false: preserve the existing automatic startup behavior.
• true: perform the normal configuration, table, sink, TSO and GC safety checks, then persist the changefeed in the existing stopped state without scheduling replication.
• Initialize the checkpoint to the validated start-ts and retain normal paused-changefeed GC protection and TTL limits.
• Keep the changefeed stopped across restarts. The existing resume API/command starts replication from its saved checkpoint unless the caller explicitly overrides it.
• Treat this as a create-only option; configuration updates must not implicitly pause or resume an existing task.

Describe alternatives you've considered

Creating a running changefeed and immediately pausing it leaves a window in which replication can already start. A paused initial state avoids that window.

Teachability, Documentation, Adoption, Migration Strategy

API callers can add "pause": true to the creation request. CLI users can put pause = true before any section headers in their TOML file and later run cdc cli changefeed resume --changefeed-id <id>.

Existing callers do not need to change. This does not introduce a new lifecycle state or unlimited data retention.

Implementation PR: https://github.com/pingcap/ticdc/pull/6265

Start by reviewing implementation PR #6265 alongside the POST /api/v2/changefeeds entry point and the CLI TOML configuration and resume command mentioned in the issue. Done means creation can preserve the existing behavior by default, support a stopped initial state with the validated checkpoint, and leave updates and restarts unchanged.