Carry errors across the wire as JSON, not as Elixir terms

[HackTuah]

One commit, red first. Independent of PR #1 — branched from main, touches no file that PR touches.

The defect

A failed tool call returned this to the client:

"structuredContent": {"error":
  "%{reason: \"invalid arguments: missing required property: a\", tool: :t}"}

inspect/1 output on an Elixir map — map-literal syntax, a bare atom with its leading colon, Elixir-escaped inner quotes. The useful parts (which tool, which property, why) were all present and recoverable only by parsing Elixir, by a client with no reason to know what language the server is written in.

It was wrong at every protocol revision, which is why it was kept out of the negotiation slice rather than folded into it.

Red first

5 tests, 3 failures
  a validation failure carries structured fields, not an inspected map
  the human-readable content carries no Elixir syntax either
  a host's error term is carried as JSON, not as an inspected term

Green: 5/5, suite 33/33, gate exit 0.

What changed

was  "error": "%{reason: \"invalid arguments: missing required property: a\", tool: :t}"
now  "error": {"tool": "widget",
               "reason": "invalid arguments: missing required property: a"}
     "text":  "widget: invalid arguments: missing required property: a"

structuredContent carries the error as data, so a client can read a field. content carries a sentence a person can read. format_reason/1 and its inspect/1 fallback are gone, and no inspect/1 remains in server.ex.

A host's error term is carried as JSON too: a dispatch returning {:error, %{code: "upstream_timeout", retry_after: 30}} reaches the client as an object with those fields. A plain-string error survives unchanged.

One thing worth noticing in the tests

They assert the absence of Elixir syntax by pattern — map literals and bare atoms — rather than comparing against a fixed expected string. A regression of a different shape is still caught, where a string comparison would only catch this exact one.

One existing assertion used =~ against what was a string and is now an object; it now reads the reason field. That is a consequence of the shape change, not an accommodation of it.