test: render: add comprehensive ERT tests for display helper functions

Add 34 new tests covering the three helper functions extracted from
wttrin--display-weather:
- test-wttrin--validate-weather-data.el (12 tests)
- test-wttrin--process-weather-content.el (12 tests)
- test-wttrin--add-buffer-instructions.el (10 tests)

Tests follow Normal/Boundary/Error pattern and all pass.

Co-Authored-By: Claude <noreply@anthropic.com>

1 files changed, 73 insertions, 0 deletions

diff --git a/tests/test-wttrin--validate-weather-data.el b/tests/test-wttrin--validate-weather-data.el
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+++ b/tests/test-wttrin--validate-weather-data.el
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+;;; test-wttrin--validate-weather-data.el --- Tests for wttrin--validate-weather-data -*- lexical-binding: t; -*-
+
+;; Copyright (C) 2024 Craig Jennings
+
+;;; Commentary:
+;; Unit tests for wttrin--validate-weather-data function.
+;; Tests validation of weather data strings for errors and nil values.
+
+;;; Code:
+
+(require 'ert)
+(require 'wttrin)
+
+;;; Normal Cases
+
+(ert-deftest test-wttrin--validate-weather-data-normal-valid-weather-string ()
+  "Test that valid weather data string is accepted."
+  (let ((valid-weather "Weather: 20°C\nCondition: Sunny\nWind: 10 km/h"))
+    (should (wttrin--validate-weather-data valid-weather))))
+
+(ert-deftest test-wttrin--validate-weather-data-normal-multiline-weather ()
+  "Test that multiline weather data is accepted."
+  (let ((weather "┌─────────────┐\n│   Weather   │\n│    20°C     │\n└─────────────┘"))
+    (should (wttrin--validate-weather-data weather))))
+
+(ert-deftest test-wttrin--validate-weather-data-normal-weather-with-unicode ()
+  "Test that weather data with Unicode characters is accepted."
+  (let ((weather "Temperature: 20°C ☀️\nWind: 15 km/h 💨"))
+    (should (wttrin--validate-weather-data weather))))
+
+;;; Boundary Cases
+
+(ert-deftest test-wttrin--validate-weather-data-boundary-empty-string ()
+  "Test that empty string is considered valid (though unusual)."
+  (should (wttrin--validate-weather-data "")))
+
+(ert-deftest test-wttrin--validate-weather-data-boundary-whitespace-only ()
+  "Test that whitespace-only string is considered valid."
+  (should (wttrin--validate-weather-data "   \n  \t  ")))
+
+(ert-deftest test-wttrin--validate-weather-data-boundary-single-character ()
+  "Test that single character string is valid."
+  (should (wttrin--validate-weather-data "x")))
+
+(ert-deftest test-wttrin--validate-weather-data-boundary-error-lowercase ()
+  "Test that lowercase 'error' is rejected (case-insensitive matching)."
+  ;; string-match uses case-fold-search which defaults to t in Emacs
+  (should-not (wttrin--validate-weather-data "error: connection failed")))
+
+(ert-deftest test-wttrin--validate-weather-data-boundary-error-in-middle ()
+  "Test that 'ERROR' anywhere in string causes rejection."
+  (should-not (wttrin--validate-weather-data "Weather: ERROR occurred while fetching")))
+
+;;; Error Cases
+
+(ert-deftest test-wttrin--validate-weather-data-error-nil-string ()
+  "Test that nil string is rejected."
+  (should-not (wttrin--validate-weather-data nil)))
+
+(ert-deftest test-wttrin--validate-weather-data-error-uppercase-error ()
+  "Test that string containing 'ERROR' is rejected."
+  (should-not (wttrin--validate-weather-data "ERROR: Unable to fetch weather")))
+
+(ert-deftest test-wttrin--validate-weather-data-error-error-at-start ()
+  "Test that 'ERROR' at start of string causes rejection."
+  (should-not (wttrin--validate-weather-data "ERROR 404")))
+
+(ert-deftest test-wttrin--validate-weather-data-error-error-at-end ()
+  "Test that 'ERROR' at end of string causes rejection."
+  (should-not (wttrin--validate-weather-data "Network ERROR")))
+
+(provide 'test-wttrin--validate-weather-data)
+;;; test-wttrin--validate-weather-data.el ends here