Fail fast on a missing or misnamed OpenAPI path parameter

[jimador]

Problem

OpenApiOperationTool.resolvePath skips substitution silently when a path
parameter's value is null, leaving the literal placeholder in the built URI.
A learned streaming API's getShow operation (GET /shows/{id}) hit this in
a downstream consumer: called without id, the tool sent GET .../shows/{id}
to the remote as-is, which 404'd every time. Nothing in the response or the
logs pointed at the missing argument — it read as "the API has no data."

That request can never succeed. OpenAPI 3 requires in: path parameters to be
required: true, so a missing one isn't optional data, it's a call to the
wrong URL.

Fix

resolvePath throws as soon as a declared path parameter has no value, naming
the parameter, the operation, and the argument keys that were actually
supplied — enough for the caller (often an LLM re-driving the call) to correct
itself instead of retrying the same dead end.

Query and header parameters are untouched; this only tightens path parameters,
which the spec already marks required.

Tests

Three new cases in OpenApiOperationToolTest: missing id, misnamed key
(imdbId instead of id), and a correctly supplied call still building the
substituted URL unchanged. Red-first: the missing-id case was confirmed
sending the percent-encoded literal (%7Bid%7D) to the mock before the fix.
164 tests green in embabel-api-client.