Add page-as-Markdown tools; source Notion-Version per operation

Adds two tools from the public Notion API that fit agentic clients and
were missing from the spec:

- retrieve-page-markdown (GET /v1/pages/{page_id}/markdown)
- update-page-markdown  (PATCH /v1/pages/{page_id}/markdown)

They read/write page content as enhanced Markdown instead of block JSON,
which is far more token-efficient for LLMs.

These endpoints require Notion-Version 2026-03-11 while the rest of the
API stays on 2025-09-03, so Notion-Version is now sourced per operation
from the OpenAPI spec: HttpClient applies each operation's header-param
default (resolving $refs), an explicitly-configured Notion-Version still
wins, the proxy no longer hardcodes it, and the parser no longer exposes
header params as model-visible tool inputs.

Also hardens the server-based integration tests against flakiness
(root cause, not retries): a shared startTestServer/stopTestServer helper
binds an ephemeral port (0), awaits the `listening` event before issuing
requests, awaits full shutdown, and uses 127.0.0.1 to avoid localhost
IPv4/IPv6 mismatch. This removes the intermittent ECONNRESET / "socket
hang up" failures from random-port collisions and request-before-bind
races.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: ksinder

README.md

@@ -69,6 +69,17 @@ If you have hardcoded tool names or prompts that reference the old database tool
 
 ---
 
+## Page content as Markdown
+
+The server exposes two tools for working with page content as enhanced Markdown instead of block JSON, which is significantly more token-efficient for AI agents:
+
+- `retrieve-page-markdown` — Read a page's full content as Markdown (`GET /v1/pages/{page_id}/markdown`). Pass `include_transcript: true` to inline meeting-note transcripts.
+- `update-page-markdown` — Edit a page's content with Markdown (`PATCH /v1/pages/{page_id}/markdown`). Prefer `replace_content` to overwrite the whole page, or `update_content` for targeted find-and-replace edits.
+
+These endpoints require Notion API version `2026-03-11`. The server now sources the `Notion-Version` header **per operation** from the OpenAPI spec, so these tools use `2026-03-11` while the rest of the API continues to use `2025-09-03` — no configuration needed. If you set `Notion-Version` yourself via `OPENAPI_MCP_HEADERS`, your value takes precedence for every tool.
+
+---
+
 ### Installation
 
 #### 1. Setting up integration in Notion