[psram] Make schema extractable with per-variant options (#16949)

Co-authored-by: J. Nick Koston <nick@koston.org>

tests/components/psram/validate-quad.esp32-s3-idf.yaml

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+# Config-only: the ESP32-S3 supports both quad and octal. The compile test uses
+# octal; this exercises the other branch of the per-variant mode enum (quad) and
+# lets speed fall back to its 40MHz default.
+psram:
+  mode: quad

tests/components/psram/validate.esp32-idf.yaml

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+# Config-only: with no options the single-mode ESP32 resolves mode -> quad and
+# speed -> 40MHz from the per-variant defaults. Compiling adds no signal here,
+# so this only runs through `esphome config`.
+psram:

tests/components/psram/validate.esp32-p4-idf.yaml

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+# Config-only: the ESP32-P4 has a distinct value set (hex mode, 20/100/200MHz).
+# With no options it resolves mode -> hex and speed -> 20MHz, exercising the
+# P4-specific default branch of the per-variant enums.
+psram: