fix(canvas): route the canvas to the site's content store

[benpeter]

Description

This pull request prepares EW Canvas for HLX6. Incl. site panels like files, outline, layout editing etc.

Test together with adobe/da-nx#672

before

blocks/browse/da-list and blocks/edit already routed through nx2's isHlx6. blocks/canvas
did not: it built its source url against da-admin regardless of the site's store. that produced
three outcomes on a source-bus site:

• da-admin 403, shown as Not permitted
• da-admin 404, passed through as loadable, so the canvas was left blank over a document that
  exists
• da-admin 200 from a pre-migration copy, which was then edited and saved in the canvas, since
  prose.js:97 named the room ${DA_ADMIN}${pathname}

now

• buildSourceUrl is async now and routes through nx2's isHlx6. initProse names the collab
  room with that url instead of rebuilding one against da-admin
• resolveEditorDocSession takes the ctx, not a built url. it confirms sign-in first, so an
  anonymous visit skips the isHlx6 probe
• the source bus is read with nx2's daFetch. da-admin is read with da-live's own fetch, for the
  live token read and the 401 retry
• sessionErrorFromResponse reports a non-401/403 status with its code and x-error, rather than
  folding it into Not permitted
• source.uploadMedia is used across the four upload paths, and the src is taken from the
  response, per fix: upload dropped images via source.uploadMedia #1219 and fix: upload pasted base64 images via source.uploadMedia #1227
• mediaBusImage renders ./media_ from getPreviewOrigin, the origin the canvas fetches its
  cookie from, per fix(prose): render relative media_ image src from preview origin #1226
• getSourceUploadContext reads both url shapes

Related Issue

part of #1141. the media browser half is #1066.

Testing

live on a source-bus site and a legacy site:

• HEAD https://api.aem.live/{org}/sites/{site}/source/canvas-test.html was answered with 200, and
  no Not permitted was shown. the collab room was named with that same url and reported
  synced: true
• a ./media_ src is rendered from stage-preview.da.live, with the relative path left in the
  node attrs. an absolute src is untouched
• a ./media_ src is written on drop, paste, toolbar and wysiwyg replace. upload
  POST https://api.aem.live/{org}/sites/{site}/media/.canvas-test/dropped.png 200
• saved to api.aem.live, no content.da.live in the document, admin.da.live 404 for that path
  throughout
• legacy: the collab room was named on admin.da.live, and a drop was answered with 201 at
  POST https://admin.da.live/source/{org}/{site}/.canvas-test/legacy-drop.png

on localhost the dropped image is not rendered on a legacy site. the read-back is answered with
401, because content.da.live/.gimme_cookie fails with net::ERR_FAILED from a localhost origin,
so the FPO placeholder is not replaced. the same happens on origin/main, served on a second port,
so this predates the branch.

Known limitations

• [not new] a save is refused on the source bus once a second editor joins the room. da-collab comma-joins
  the connections' Authorization and helix parses one pair, so the write is unauthenticated
• [not new] isHlx6 caches a legacy answer in module memory for the life of the tab (api.js:629). a site
  migrated mid-session is read as legacy until reload
• nx2's fetcher is used for the doc-pane uploads. it snapshots the token at page load and does not
  retry a 401. checkDoc uses da-live's own fetcher instead
• [not new] canWrite is optimistic on the source bus. status.get would answer it, at one request per
  open

Test:

• https://ewhlx6--da-live--adobe.aem.live/canvas?nx=hlx6cfg#/exp-workspace/frescopa6/index

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Commits

• cf312ff ✅ (latest)
• 8ea1ff1 ✅
• 5f44f11 ✅
• cd558cc ✅
• 2897858 ✅
• 2e70997 ✅
• 85aaa38 ✅
• 0b6b765 ✅
• 605a7fe ✅
• 57e8f95 ✅
• 94601ea ✅
• 5b7053a ✅
• b9ec78d ✅
• 79d4c04 ✅
• 7b68d91 ✅
• a292029 ✅
• cf09f96 ✅
• 6536d15 ✅
• 95fa1fb ✅
• dc47d59 ✅

[mhaack]

LGTM as well, as discussed we will need to add the toast for the image upload failures. Otherwise 👍