fix(graphs): return the extension display name instead of dropping it

[jfrench9]

getAvailableExtensions builds a display-names table, looks each extension up
in it, puts the result in a dict — and then drops it when constructing the
AvailableExtension response objects, which carry only name, description
and enabled:

extension_objects = [
  AvailableExtension(
    name=str(ext["name"]), description=str(ext["description"]), enabled=False
  )
  for ext in available_extensions   # ext["display_name"] computed, never read
]

So the only human-readable label the endpoint has never leaves it. Clients get
name, which is the schema slug the create path takes, and the visible
consequence is the graph creation wizard heading its extension cards
"roboledger" and "roboinvestor" — plus the required-extensions alert listing
slugs.

Found while fixing dark-mode issues in that same wizard
(robosystems-app#351).

Change

• display_name added to AvailableExtension, optional so an older client is
  unaffected; clients fall back to the slug when it is absent.
• Populated on both response paths. The fallback branch never had one at all,
  so the table is hoisted to module scope beside the descriptions, where the
  existing comment already explains why those constants are single-sourced.
• Test asserts the display name survives into the response. Verified it fails
  against the old source (KeyError), so it is not vacuous.

Follow-up (not in this PR)

The apps can't use the field until the TypeScript client is regenerated and
released, then adopted. Happy to take that chain next if you want it.

just test-code clean; pytest tests/routers/graphs tests/models green
(1735 passed, 4 skipped).