fix(sequentialthinking): keep nextThoughtNeeded in inputSchema required

[Golchic]

Description

nextThoughtNeeded is declared as a non-optional field on the sequentialthinking
tool, but it is missing from the generated JSON Schema's required array. Clients
that build call arguments from the advertised schema omit it and get
-32602 Input validation error, which contradicts the schema.

Root cause: the coercedBoolean helper was z.preprocess(fn, z.boolean()). Under
Zod v4, z.preprocess widens the input type to unknown, so the SDK's JSON
Schema generation (io: "input") treats the field as optional and drops it from
required — even though it is not .optional(). (Regression from #3533.)

Fix: reimplement coercedBoolean as z.union([z.boolean(), z.string().transform(...)]).
The union keeps a concrete input type, so non-optional fields stay in required,
while still coercing "true"/"false" (case-insensitively) and now rejecting other
strings with a clearer message. isRevision / needsMoreThoughts remain optional.

The only schema shape change: these boolean fields now serialize as
{ "anyOf": [{ "type": "boolean" }, { "type": "string" }] }, which accurately
reflects the accepted input.

Server Details

• Server: sequentialthinking
• Changes to: tools (input schema for sequentialthinking)

Motivation and Context

Fixes #4651. Without this, schema-driven MCP clients cannot call the tool without
manually knowing nextThoughtNeeded is required despite the schema saying otherwise.

How Has This Been Tested?

• New src/sequentialthinking/__tests__/schema.test.ts:
  • Round-trips a probe tool through the MCP SDK (McpServer + Client over
    InMemoryTransport) and asserts a required coercedBoolean field stays in
    inputSchema.required, while an .optional() one does not.
  • Value coercion: true/false, "true"/"false"/"TRUE"/"False", and
    rejection of "yes", "", 1, null.
• npm run build and npm test pass in src/sequentialthinking (19 tests).
• Verified against the SDK's actual conversion path (Zod v4 → zod/v4-mini
toJSONSchema, io: "input"), not a hand-rolled converter.

Breaking Changes

None for tool callers — the field was always meant to be required and the runtime
validation already required it. MCP client configs need no changes.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist

• I have read the MCP Protocol Documentation
• My changes follows MCP security best practices
• I have updated the server's README accordingly — not needed; the README
  already lists nextThoughtNeeded as a non-optional (boolean) input
• I have tested this with an LLM client — tested with the MCP SDK client and
  automated tests
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have documented all environment variables and configuration options — n/a

Additional context

coercedBoolean moved from index.ts to lib.ts (exported) so it can be unit
tested; index.ts now imports it. zod is added as an explicit dependency of the
package since it is imported directly (matching the everything server).