fix: honor a defined-but-empty value, and announce the expansion error

Addresses Copilot's round-8 review on #2035 (one inline comment, one
suppressed). Both real.

1. RFC 6570 distinguishes an UNDEFINED variable, which omits its whole
   expression, from one defined as the empty string, which expands. The
   expander was calling `definedValues` on the way in, collapsing the two --
   so `x{?q}` with `{ q: "" }` gave `x` instead of `x?q=`, and there was no
   way for any caller, `readResourceFromTemplate` included, to request
   either that or `x;q`. Key presence is now honored, including §3.2.7's
   rule that the matrix operator drops the `=` for an empty value (`x{;q}`
   -> `x;q`, while `?`/`&` keep it).

   Dropping blanks is a FORM concern, so it moved to the two form
   boundaries: the web panel seeds every declared variable with "" and the
   TUI's ink-form returns "" for every untouched field, so neither can tell
   "never touched" from "deliberately empty" -- but that is a fact about a
   text input, not about the template, and it has no business deciding what
   a template means for every other caller.

   Six unit tests and one integration test encoded the old collapse and now
   assert the corrected meaning. The prototype-collision cases get stronger
   rather than weaker: a blank `{?toString}` expands to `?toString=` -- its
   OWN empty value -- where the original bug expanded the inherited
   function's body.

2. The expansion error is now `role="alert"`. It can appear while typing (a
   pasted value that cannot be encoded), and plain text is not announced, so
   a screen-reader user met a disabled Read Resource with no stated reason.

npm run ci passes.

Signed-off-by: cliffhall <cliff@futurescale.com>

AGENTS.md

@@ -95,7 +95,11 @@ v2/main/
 │   │                                   #   tryExpandUriTemplate returns the reason as a value, and
 │   │                                   #   the panel disables Read Resource rather than sending the
 │   │                                   #   raw template — expandUriTemplate's raw-template fallback
-│   │                                   #   is for DISPLAY (the preview) only — #1919;
+│   │                                   #   is for DISPLAY (the preview) only. An UNDEFINED
+│   │                                   #   variable omits its expression; one defined as ""
+│   │                                   #   expands (`x{?q}` → `x?q=`), so each FORM — not the
+│   │                                   #   expander — drops its untouched blanks via
+│   │                                   #   definedValues — #1919;
 │   │                                   #   modernTaskSchemas.ts: SEP-2663 modern Tasks
 │   │                                   #   extension wire schemas + normalize/handle helpers,
 │   │                                   #   used by the raw-wire tasks/* channel — #1631;