Warn on implicit stdio initialization

[atesgoral]

Summary

Implements phase 2 of #334 after the 0.15.0 release.

• Emit a deprecation warning when MCP::Client::Stdio#send_request performs the compatibility implicit initialization path.
• Keep the existing implicit initialization behavior for this release step.
• Cover the warning path and the explicit MCP::Client#connect no-warning path.
• Add an Unreleased changelog entry.

How Has This Been Tested?

• dev test -- 847 runs, 2159 assertions, 0 failures

Notes

dev style currently reports existing unrelated Minitest/AssertEmptyLiteral offenses in files outside this change. No new offense is reported for the touched files.

[koic]

Thanks for the follow-up. Since v0.15.0 was just released, wait a bit before merging and releasing.

[atesgoral]

@koic Shall we get this in now?

[koic]

The timing for merging and releasing this Phase 2 is a bit of a balancing act, but the idea was to merge it after roughly a month had passed since the release of https://github.com/modelcontextprotocol/ruby-sdk/releases/tag/v0.15.0, which already included Phase 1.

At the same time, delaying it too long would reduce the chance for existing users to become aware of the transition, so it probably makes sense to merge it fairly soon.

On the other hand, the final behavior change in Phase 3 will be a breaking change, so it may be better to leave a reasonably long gap after Phase 2 before shipping that one (perhaps around three months?).

In any case, it probably makes sense to include this in the next release. Thank you!