Previous 2024.2.4 SDK was a leftover from initial plugin scaffolding —
nobody targets that build in practice. Bumping the SDK to 2025.2 (the
strykelang baseline) so the plugin compiles against the current
module-split platform layout that real RustRover / IDEA installs use.

Changes:
* gradle.properties: platformVersion 2024.2.4 → 2025.2,
  pluginSinceBuild 242 → 252 (untilBuild 261.* unchanged, covers
  2026.1.1).
* build.gradle.kts: kotlin.jvm 2.0.21 → 2.2.0 (2025.2 platform classes
  carry Kotlin 2.2 metadata; 2.0.x compiler can't read them).
* build.gradle.kts: add `bundledModule("intellij.spellchecker")` —
  2025.2 ships spellchecker as a platform MODULE
  (lib/modules/intellij.spellchecker.jar) rather than embedded.
* plugin.xml: re-add `<depends>com.intellij.spellchecker</depends>` —
  required so the `spellchecker.support` EP registration wires up at
  runtime. Now valid because 2025.2+ exposes spellchecker as a
  separately-resolvable module ID (which was the verifier error on
  the previous 2024.2.4 SDK).

Net effect: TypoInspection is finally suppressed inside STRING_DQ /
HEREDOC / COMMENT tokens on real RustRover 2025.2 / 2026.1.1.