fix: stop returning snapshot filter errors to the model as page content

[karaposu]

Problem

When the filtered ARIA snapshot path fails to parse, Aria_snapshot_filter.filter_snapshot's catch
returns Error filtering snapshot: <message>\n<stack> in the same slot as the page's interactive
elements. The snapshot tool prints that string verbatim under its Interactive Elements: header — so
the model receives a JavaScript stack trace presented as the page, with no signal it's an error, and
reasons about garbage. The realistic trigger is Playwright's ariaSnapshot returning a non-string
shape (the same API drift #158/#160 dealt with).

Fix (two changes, atomic by design)

• filter_snapshot no longer catches: a parse failure throws. The
  'No interactive elements found' empty case stays — that is a value, not an error.
• capture_snapshot wraps only the filter call. On a throw it logs the full stack to stderr and
  returns the unfiltered snapshot — real content already in hand — prefixed with a model-visible
  [note: snapshot filtering failed (…)] line, in the same result shape as filtered:false, so the
  tool renders it with no changes. Refs from the full snapshot still resolve through ref_locator's
  aria-ref branch, so the click/type workflow keeps working after a fallback. A non-string snapshot
  shows (no snapshot text available) instead of the word undefined.

Why this matters for the filtered:true default

This also makes flipping the snapshot tool's filtered default to true (as #163's bb37007 does)
safe: with the default on, every snapshot rides the filter path, and a filter bug would otherwise sit
on the main road. Merge note: bb37007 edits the same capture_snapshot function (DOM-ref
renumbering) — the changes are logically independent.

Tests

test/aria-snapshot-filter.test.js: the parser/format matrix (verified against the real parser
before being written), the throw case (fails on the pre-fix code, which returned the error string),
and a fallback wiring test using an injected fake page — asserting note + full content, no DOM pass,
and an intact result shape.