docs: derive example call values from schema example/default/enum

[andig]

The Markdown doc generator (cmd/openapi-mcp/doc.go) built example-call values from the parameter type alone, emitting "example"/true placeholders even when a param defined example, enum or default.

exampleValue now prefers, in order: example → default → first enum value → the type-based placeholder. Params without any of those keep the previous placeholder behaviour. The runtime tool schema is unchanged — only generated docs are affected.

Verified against evcc server/openapi.yaml, getEnergyHistory:

call getEnergyHistory {
  "aggregate": "15m",     // was "example"  (default)
  "format": "json",       // was "example"  (enum/default)
  "grouped": false,       // was true       (default)
  "from": "example",      // unchanged: no example/default/enum
  "lang": "example",
  "to": "example"
}

Adds doc_test.go covering the precedence.

Fixes evcc-io/evcc#31377

🤖 Generated with Claude Code

[naltatis]

LGTM