scaffolder: Support conditional if on output links and text (#33332)

* Add sample template for conditional output demo

Add a scaffolder template that exercises conditional `if` on output
links and text items, for testing issue #24805.

Signed-off-by: Dmitry Gusev <gusevda90@gmail.com>

* Support conditional `if` on scaffolder output links and text

Add `if` property to output link and text items in scaffolder templates,
allowing template authors to conditionally show/hide output items based
on parameters or step results. Items with a falsy `if` condition are
filtered out before being sent to the frontend.

Signed-off-by: Dmitry Gusev <gusevda90@gmail.com>

* Add changesets for conditional output feature

Signed-off-by: Dmitry Gusev <gusevda90@gmail.com>

* Document conditional if on output links and text

Signed-off-by: Dmitry Gusev <gusevda90@gmail.com>

* Guard against non-object items in output arrays

Signed-off-by: Dmitry Gusev <gusevda90@gmail.com>

* Skip array items in output if-filtering destructuring

Add Array.isArray guard to prevent corrupting array items
into plain objects during the rest-destructuring step.

Signed-off-by: Dmitry Gusev <gusevda90@gmail.com>

* Extract filterConditionalItems helper to deduplicate logic

Signed-off-by: Dmitry Gusev <gusevda90@gmail.com>

* Align output if schema descriptions with step if semantics

Signed-off-by: Dmitry Gusev <gusevda90@gmail.com>

* Fix docs quality check: replace 'falsy' wording

Signed-off-by: Dmitry Gusev <gusevda90@gmail.com>

* Refactor filterConditionalItems to use flatMap and generics

Replace .filter().map() with a single flatMap call and make the function
generic to eliminate JsonArray casts at call sites.

Signed-off-by: Dmitry Gusev <gusevda90@gmail.com>

---------

Signed-off-by: Dmitry Gusev <gusevda90@gmail.com>

.changeset/metal-parts-doubt.md

@@ -0,0 +1,5 @@
+---
+'@backstage/plugin-scaffolder-common': patch
+---
+
+Added optional `if` property to `ScaffolderOutputLink` and `ScaffolderOutputText` types, allowing template authors to conditionally include output links and text items.

.changeset/silver-mangos-think.md

@@ -0,0 +1,5 @@
+---
+'@backstage/plugin-scaffolder-backend': patch
+---
+
+Added support for conditional `if` filtering on output `links` and `text` items. Items where the `if` condition evaluates to false are now excluded from the task output.