Support in-place publishing page transformation

[TaoziZ03]

Publishing pages can be transformed to modern Site Pages in the source Web or in a supported target in another site collection. The transformation mode is determined from the actual Site and Web identities.

In-place transformation is an explicit opt-in through the new PublishingPageTransformator.CreateInPlace factory overloads, which supply the source context as the target. Both existing constructors still reject a null targetClientContext, so current callers keep their present behavior.

Same-Web conversion requires an existing writable Site Pages library, retains the source publishing page, and rejects existing targets and inconclusive collision probes before the first target-page operation. Different Webs within the same site collection remain outside the supported target scope. Cross-site permission, naming, and overwrite behavior is preserved.

A missing target page is signaled with the new PageDoesNotExistInWebException instead of matching LogStrings message text. Those constants are English-only and the file documents them as pending localization, so a text match would have silently stopped in-place collision routing. The new type derives from ArgumentException, so existing handlers that test for ArgumentException are unaffected.

Image mappings emit numeric -1 defaults when file dimensions are absent or the referenced file cannot be found. Missing files retain their original references and produce a warning instead of leaving unresolved numeric mapping tokens. This lets conversion continue; it does not restore a missing image.

Modernization loads root-Web metadata through an independent CSOM object query. This avoids a service-side target-null failure observed when RootWeb is expanded through Site.

PowerShell consumer: pnp/powershell#5490. That PR still constructs the transformator with a null target when no target parameter is supplied, and needs a follow-up change to call CreateInPlace instead. It must not merge before this change ships in a PnP.Framework release.

No CHANGELOG.md entry is included: src/lib/CHANGELOG.md states that the file is maintained by the repo owner.

Validation

• Builds passed for netstandard2.0, net8.0, net9.0, and net10.0: 0 errors; existing repository/compiler and NuGet warnings remain.
• The focused PublishingPageInPlaceTransformationTests and BuiltInImageLookupTests suite passed: 27 passed, 0 failed, 0 skipped. Coverage includes the CreateInPlace opt-in, null-target rejection by both constructors, the collision and rejected-target seams, and the missing-file case validating numeric JSON in both image mappings.
• Existing ArgumentException call sites were re-checked against the new exception type: PageTransformator still matches it through ex is ArgumentException, and the CrossSiteTests overwrite assertions continue to observe a plain ArgumentException carrying the existing message.
• Live validation below was executed before the constructor opt-in was introduced. CreateInPlace produces the same source-as-target state those runs exercised, so the observed in-place behavior is unchanged; the constructor change only affects callers that passed an explicit null.
• Live Enterprise Wiki validation passed for same-Web conversion without target parameters and cross-site conversion through both -TargetWebUrl and -TargetConnection. Native editor save/exit passed.
• Live non-Enterprise-Wiki validation covered three existing Publishing pages, each in both modes: a rich ArticleLeft page, a simple ArticleLeft page with a List web part, and a custom IPKit_Home layout. All six generated native modern drafts and completed editor save/exit.
• The rich ArticleLeft page initially failed in-site because its three image URLs return 404 and missing image lookup left numeric tokens unresolved. With the fix, its 31 checked content blocks, 8 tables, and 3 Image web parts persisted. Missing images and source document embeds remain unresolved dependencies.
• IPKit_Home needs an explicit -PageLayoutMapping on this tenant. Its catalog item lacks PublishingAssociatedContentType, preventing automatic layout analysis. A locally validated mapping bound to PublishingPageContent and the actual Top/Bottom/Usage/Right zones converted both modes; all 16 checked body blocks persisted. The mapping is validation input, not a new built-in template.
• Cross-site conversion retained existing skip behavior when corresponding target lists were absent: 1 List web part on each ArticleLeft fixture and 4 on IPKit_Home. Same-Web retained them. Script Editor behavior remains the existing default.
• Original source-file/content hashes, identities, and versions remained unchanged. Six non-wiki drafts were read back at version 0.3. Failed mapping cases left no target page.
• Existing-target -Overwrite, missing Site Pages, and different-Web/same-site-collection negative cases were rejected without changing source/target files.
• The Enterprise Wiki SVG fixture retains its readable source reference; SVG asset independence is not claimed.
• git diff --check passed.

Live acceptance is bounded to the tested tenant, existing writable Site Pages libraries, the stock Enterprise Wiki/ArticleLeft layouts, and the supplied IPKit_Home mapping. Draft creation and editor persistence do not establish complete asset, list, script, or visual fidelity.