refactor(chat,#1158): DRY adapter base default renderMessageElement

[joelteply]

Summary

Lifts the byte-identical renderMessageElement body from three subclasses
(TextMessageAdapter, URLCardAdapter, ToolOutputAdapter) into
AbstractMessageAdapter as the new default. Subclass overrides are deleted;
ImageMessageAdapter retains its custom override (it builds <img> nodes
via property assignment to keep src/alt out of any HTML-parse path and
does not go through renderContent to string).

Net -61 LOC.

Pattern

"ALWAYS look for patterns and formulate them into design ... when you see
3+ similar things, find the primitive."

Three subclass overrides did the same thing:

parseContent(message) -> data
createAdapterWrapper() -> wrapper
renderContent(data, currentUserId) -> contentHtml
const template = document.createElement('template')
template.innerHTML = contentHtml
wrapper.appendChild(template.content.cloneNode(true))
return wrapper

That is now the base default body. The detached-template path (so the live
message-content slot never sees innerHTML directly) is preserved
identically -- Lit-managed reactive children still survive sibling updates.

ImageMessageAdapter is the natural-different outlier that proves the
abstraction is right: it doesn't fit the string-render pattern, so it stays
as a custom override. Three things compress to a primitive; the fourth
proves the primitive is real.

What's in the PR

• AbstractMessageAdapter.renderMessageElement -- replaced return null
  default with the full DRY body. Class-name in error log resolves via
  this.constructor?.name so each subclass logs under its own name.
• TextMessageAdapter -- override deleted; one-line comment points at
  base default.
• URLCardAdapter -- override deleted; one-line comment points at the
  XSS-hardening follow-up (#1159).
• ToolOutputAdapter -- override deleted; one-line comment notes that
  escapeHtml at renderContent interpolation sites still applies.
• eslint-baseline.txt -- ratcheted -2 (deleted code carried 2 baseline
  errors).

Why ImageMessageAdapter stays custom

ImageMessageAdapter.renderMessageElement builds the wrapper's children
via DOM APIs (document.createElement('img') + img.src = url / img.alt = ...)
so the src/alt values never pass through any HTML parse step. That's
intentional defense-in-depth for the media path; the base default's
template.innerHTML = contentHtml would re-parse the image markup, which
is the exact thing this adapter was written to avoid. Keep it custom.

Tests

• npm run build:ts -- clean.
• Pre-commit hook -- TypeScript + browser ping passing.
• Pre-push hook -- ESLint baseline ratcheted -2; no new violations.
• No existing tests assert renderMessageElement shape directly; the
  adapter unit suite is a sub-card (see precommit gate too thin: only browser-ping runs; no chat-roundtrip / adapter unit / persona-reply smoke #1186).

Followups

• feat(chat,#1100): URL-metadata XSS hardening (PR-3) #1159 -- URL metadata XSS hardening (escape at renderContent
  interpolation sites for originalText, title, description,
  siteName). Now visibly called out in the URLCardAdapter comment.
• precommit gate too thin: only browser-ping runs; no chat-roundtrip / adapter unit / persona-reply smoke #1186 -- Precommit gate too thin -- adapter unit tests should
  fire when widgets/chat/adapters/ changes.

Closes #1158.