docs: fix snippet-source inaccuracies and regenerate

[jeremydmiller]

What

Second follow-up from the docs audit. These defects live inside generated snippet blocks, so the fix is in the source #region markers; the markdown is then regenerated with mdsnippets. Source changes are comment-only except where noted.

Fixes

• diagnostics / sample_using_request_count — sync Count()/Any() → CountAsync()/AnyAsync(). Synchronous query execution was removed in Marten 9 (these would throw NotSupportedException). Request count is unchanged (3).
• linq/operators / sample_select_a_single_value — removed Last()/LastOrDefault() from the element-operations sample. Marten throws InvalidOperationException on them; the guidance is to reverse the ordering and use First()/FirstOrDefault(). Comment updated accordingly.
• events/appending / sample_configuring_event_append_mode — comment claimed Rich is the default "from 4.0 on"; the default is QuickWithServerTimestamps as of Marten 9.
• events/subscriptions / sample_errorhandlingsubscription — AfterCommitAsync and BeforeCommitAsync log messages were swapped.
• multi-stream + optimizing / sample_showing_fanout_rules — cache comment said "up to 100" but the value is 1000.
• computed-indexes + unique / sample_customizing-calculated-index — copy-paste "Change the index method to brin" comment sat over a SortOrder.Desc assignment.
• schema/authorization / sample_customizing_table_creation — comment implied DropThenCreate is the default; CreateIfNotExists is the default.

Verification

mdsnippets regenerated the affected markdown (incidental snippet source-link line-number refreshes ride along). Marten.Testing builds clean (the only non-comment source edits — CountAsync/AnyAsync and the removed Last() lines — are there).

🤖 Generated with Claude Code