feat(expect/unstable): implement toThrowErrorMatchingSnapshot()

[anyingiit]

Description

Implements expect(fn).toThrowErrorMatchingSnapshot(hint?), one of the two APIs that expect/mod.ts still lists as unavailable (tracking issue #3964 asks for one PR per API, so the inline variant is left for a follow-up).

Behaviour follows Jest:

• the received function is called and the message of the thrown error is compared with the stored snapshot (-- --update writes it, exactly like toMatchSnapshot());
• with .rejects, the rejection reason's message is used;
• if the function does not throw, it fails with Received function did not throw;
• .not is rejected with the same error as the other snapshot matchers;
• it shares the per-test snapshot counter with toMatchSnapshot(), so both can be mixed in one test.

To avoid duplicating the snapshot bookkeeping, the body of toMatchSnapshot() (test path/name lookup, key/counter, assert vs. update mode) moved into a private matchSnapshot() helper that both matchers call. toMatchSnapshot() behaves as before, including the order of its checks; only the matcher name in the "Unable to determine test file path/name" errors is now parameterised. The new matcher is marked @experimental like toMatchSnapshot(), and mod.ts now lists it as supported.

Motivation / disclosure: I had some spare AI-assistant quota (Claude Code) and am using it to try to
help projects with open good-first-issues. The change was prepared with Claude Code and verified as listed
below. If it doesn't fit, isn't up to your bar, or you'd simply rather not take it — please feel free to
close it, no hard feelings at all 🙂

Related issue

Part of #3964

Checklist

• Tests pass locally (deno test -A --doc --parallel --trace-leaks expect/ → 237 passed, 0 failed; the new expect/_to_throw_error_matching_snapshot_test.ts has 9 tests that all fail on main and pass with this change; deno lint, deno fmt --check, deno task lint:docs, lint:mod-exports, lint:export-names, lint:unstable-deps, lint:circular and test:browser all pass)
• CHANGELOG.md is updated (if applicable) — n/a, release notes are generated from the PR title
• Documentation is updated (if applicable) — JSDoc on Expected.toThrowErrorMatchingSnapshot and the supported/unsupported list in expect/mod.ts

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

❌ Patch coverage is 93.75000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.04%. Comparing base (2958335) to head (9e31769).

Files with missing lines	Patch %	Lines
expect/_matchers.ts	93.47%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #7332    +/-   ##
========================================
  Coverage   95.04%   95.04%            
========================================
  Files         619      618     -1     
  Lines       52012    51799   -213     
  Branches     9450     9411    -39     
========================================
- Hits        49433    49231   -202     
+ Misses       2031     2023     -8     
+ Partials      548      545     -3     

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