fix: walk contentSchema, await the error envelope, correct the README claim

Copilot review round 7.

- Add `contentSchema` to the traversal. It holds the schema the decoded
  string content must satisfy, so a bare `true` or a remote `$ref` under
  it is a real finding all three clients were walking past. Also admit
  the three content keywords to `CONSTRAINING_KEYWORDS` on the same
  documented grounds as `format` — annotation-only by vocabulary, but
  plainly an attempt to constrain, so reporting such a schema as
  accepting anything would be noise. The comment now names that
  category rather than singling `format` out.

- Make `handleError` async and await the envelope write. Round 6 made
  the `--strict` report await its write and stopped one line short: the
  envelope emitted for the very same throw still raced `process.exit()`,
  so the machine-readable line a CI caller parses could be lost on a
  pipe. The launcher's comment claiming the envelope was "well inside
  the pipe buffer" was wrong reasoning and is replaced — fitting in the
  buffer says nothing about whether the write was performed. Its call
  site now awaits; the bin's `.catch(handleError)` needed no change.

- The CLI README called its sample block the "complete output" when the
  error envelope follows it on every non-zero exit. Now scoped to "the
  complete report", with the envelope named and linked.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UL9retfvAXRgvi6EWk4SY4
Signed-off-by: cliffhall <cliff@futurescale.com>

clients/cli/README.md

@@ -164,8 +164,10 @@ on **stderr**, and exits `6` if any is error-severity:
 mcp-inspector --cli <server> --method tools/list --strict
 ```
 
-Complete output against the `unportable-schemas-http.json` showcase — one
-block per finding, then the summary:
+The complete **report** against the `unportable-schemas-http.json` showcase —
+one block per finding, then the summary. On a non-zero exit the shared error
+handler adds one more stderr line after this, the
+[`ErrorEnvelope`](#exit-codes--error-envelopes) (`{"error":{"code":"schema_unportable",…}}`):
 
 ```text
 Error: tool "get_temp"