chore: Simplify verbose C# code with LINQ and idiomatic patterns

## What

Several methods use verbose imperative loops where idiomatic LINQ or shorter patterns apply.

## Changes

### `ChangelogTextUtilities` — collapse parallel helper pairs
`FormatPrLink`/`FormatIssueLink` and their `*Asciidoc` twins, plus `ExtractPrNumber`/`ExtractIssueNumber`, differ only by the URL segment (`"pull"` vs `"issues"`). Collapse each pair into one method with a parameter.
File: `src/Elastic.Documentation/ReleaseNotes/ChangelogTextUtilities.cs:168-229,322-441` (~60→20 lines)

### `ChangelogBundlingService.ApplyGlobalContentBundleFilter` → `.Where()`
Manual `new List<>` + `foreach` + multiple `if...continue` guards. Replace with `.Where()` and a combined predicate.
File: `src/services/Elastic.Changelog/Bundling/ChangelogBundlingService.cs` (~30→6 lines)

### `OpenApiGenerator` classification → `.GroupBy()`
Nested `foreach` + `List.Add()` manual grouping. Replace with `.GroupBy().Select()`.
File: `src/Elastic.ApiExplorer/OpenApiGenerator.cs:~119-163` (~25 lines)

### `Move.cs` dict accumulation → `TryAdd`
`if (!dict.ContainsKey(k)) dict[k] = []` (two sites). Replace with `dict.TryAdd(k, [])`.
File: `src/authoring/Elastic.Documentation.Refactor/Move.cs:78,323`

### `StreamTransformerFactory` + `AskAiGatewayFactory` → direct resolution
Both are service-locator shims with a single call site that dispatch to one of two impls by config string. Collapse to a conditional resolver in the call site or use DI keyed services.
Files: `src/api/Elastic.Documentation.Api/Adapters/AskAi/StreamTransformerFactory.cs` (~47 lines), `AskAiGatewayFactory.cs` (~33 lines)

### `CodexPageViewFactory` → inline into caller
Pure delegation wrapper with one call site; sets two properties then calls `Index.Create()`. Inline into `CodexBuildService`.
File: `src/Elastic.Codex/Page/CodexPageViewFactory.cs` (~23 lines)

### `AddLlmMarkdownExport` extension → inline into caller
One-line duplicate of the `new LlmMarkdownExporter(branded)` logic already in `CreateMarkdownExporters`; single caller at `ReloadableGeneratorState.cs:104`.
File: `src/Elastic.Markdown/Exporters/LlmMarkdownExporter.cs:227` (~3 lines)

~225 lines removed across these changes.

Part of the broader ponytail over-engineering audit tracked in #3574.