fix(sequentialthinking): correct readOnlyHint and idempotentHint annotations

[somuai]

Resolves #4721.

Problem

The sequentialthinking tool annotations previously specified readOnlyHint: true and idempotentHint: true. However:

1. The server maintains per-session instance state (thoughtHistory and branches), which mutates on every processThought invocation. It is therefore stateful, not read-only.
2. Repeated calls with identical arguments yield different thoughtHistoryLength values (and mutate branches), making the tool non-idempotent by definition. Clients relying on readOnlyHint (e.g. for parallel execution safety) or idempotentHint (e.g. for caching or automatic retries) would encounter unexpected side effects.

Solution

• Set readOnlyHint: false and idempotentHint: false in src/sequentialthinking/index.ts.
• Added regression test in src/sequentialthinking/__tests__/input-schema.test.ts verifying that advertised tool annotations accurately reflect statefulness.

[cliffhall]

Thanks for this contribution! #4721 has been resolved on the v2/main line by #5015, so I'm closing this PR as superseded. If anything is missing from that fix, please comment on #4721 or open a PR against v2/main.