P2: no status pinned for wrong Content-Type -- pin 415, register unsupported_media_type

[ssilvius]

Severity: P2. Reviewed at 82b715c.

S2 (SPEC.md:24) requires Content-Type: application/json on requests but never says what a server does when it gets something else. The conformance suite shrugs at the gap rather than closing it:

| R-4 | Content-Type other than application/json | 400 or 415 |

"400 or 415" is not a conformance assertion -- a client cannot branch on it, and two conforming servers can disagree.

Fix: pin 415, register unsupported_media_type

415 is the correct HTTP status for this condition, and none of the eight currently registered error codes fits: bad_request is about the request body, and conflating a malformed body with an unacceptable media type loses the distinction a client needs to retry correctly (fix your headers vs. fix your payload).

Per S11, adding an error code is an additive minor change, so this is in scope for a v0.1.x revision rather than requiring a major.

Acceptance

• S2 states the required response for a non-JSON Content-Type: 415 with error.code = unsupported_media_type.
• unsupported_media_type added to the S7 registered code table with HTTP 415.
• Conformance R-4 expects 415 specifically, not "400 or 415".
• Both reference servers emit it.

Origin: codex spec review, verified against SPEC.md:24 and conformance/README.md R-4.