fix: advertise JSON Schema 2020-12 in tools/list

[rbuchmayer-pplx]

Fixes #132.

Problem

tools/list advertises $schema: http://json-schema.org/draft-07/schema# on every input and output schema. Clients that only support JSON Schema 2020-12 (the dialect the MCP spec specifies) reject all 8 schemas before any tool is invoked, so every tool is unusable in those clients.

Root cause is in the MCP TypeScript SDK: the zod v3 conversion path never sets a dialect target, so zod-to-json-schema defaults to draft-07 (modelcontextprotocol/typescript-sdk#2084). The upstream fix (modelcontextprotocol/typescript-sdk#2085) is not merged or released yet, so this patches it on our side.

Fix

The schemas themselves are valid under both dialects (verified with a strict Ajv 2020-12 validator: 8/8 rejected as-is, 8/8 compile after changing only the $schema string; no draft-07-only constructs present). So createPerplexityServer now wraps the SDK's tools/list handler and rewrites the declared dialect to 2020-12. This mirrors what the upstream fix does for zod v3 schemas. The shim touches a private SDK field and is meant to be removed once the SDK fix ships and we bump past it.

Version goes to 1.2.1 so the fix publishes on merge.

Verification

• New test: in-memory client calls tools/list and asserts 2020-12 on all input and output schemas. Fails without the fix, passes with it.
• Full suite: 90/90 passing, npm run build clean.
• Manually confirmed against a strict 2020-12-only Ajv validator: all 8 schemas compile after the fix.