fix(photon): stop advertising fenced code-block support on iMessage

The gateway's tool-progress path emits terminal commands as fenced code
blocks on any adapter whose supports_code_blocks is True. The Photon
adapter set that flag from PHOTON_MARKDOWN, but the sidecar's /send
router (send-format.mjs) silently routes every URL-bearing message
through the plain-text builder — where fences survive as literal backtick
characters — and even the markdown path renders a fence as inline
monospace text, not a block. Net effect: raw fenced terminal commands
(and raw markdown around them) surfaced in iMessage bubbles no matter
what the user's prompt-level style rules said.

Fix the whole class: the adapter never claims code-block support, so the
gateway emits its compact one-line tool preview instead. Prose markdown
passthrough (bold/italic/headings) is unchanged, and no config change is
required — tool_progress can stay at its user's preferred mode.

Tests: new capability + E2E tests pin that the gateway cannot emit a
fence for Photon with real adapter + real display resolution; the old
supports_code_blocks-mirrors-env expectation (which pinned the buggy
behavior) now asserts the flag is always False.

tests/plugins/platforms/photon/test_markdown.py

@@ -43,9 +43,16 @@ def test_format_message_passthrough_by_default(
     assert adapter.format_message(_MD) == _MD
 
 
-def test_supports_code_blocks_mirrors_env(monkeypatch: pytest.MonkeyPatch) -> None:
+def test_supports_code_blocks_never_enabled(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    """Fenced code blocks are not renderable on iMessage: URL-bearing
+    messages fall back to raw text (literal fences), and the markdown path
+    renders a fence as inline monospace, not a block. The adapter therefore
+    never advertises code-block support — the gateway emits its compact
+    one-line tool preview instead (see test_code_block_capability.py)."""
     monkeypatch.delenv("PHOTON_MARKDOWN", raising=False)
-    assert _make_adapter(monkeypatch).supports_code_blocks is True
+    assert _make_adapter(monkeypatch).supports_code_blocks is False
     monkeypatch.setenv("PHOTON_MARKDOWN", "false")
     assert _make_adapter(monkeypatch).supports_code_blocks is False