Don't nilify non-string values in case_insensitive_keys / strip_whitespace_keys

[MahmoudBakr23]

What

When a non-string value (e.g. an integer column) is listed in config.case_insensitive_keys or config.strip_whitespace_keys, the value is silently overwritten with nil during before_validation.

Why

Authenticatable#apply_to_attribute_or_variable applies :downcase / :strip via .try(method). Object#try returns nil when the receiver doesn't respond to the method, and the result is then assigned back to the attribute. So an integer column listed in one of these keys gets nil-ified on every save attempt.

This was reported in #5780, where a typo in the config (%i[email, site_id], which parses to [:"email,", :site_id]) caused :site_id — an integer column — to end up in the list and silently get reset to nil. The misconfiguration is on the user side, but the failure mode is silent data loss rather than a no-op or a clear error, which is what makes it hard to debug (the reporter spent an hour tracing it).

Fix

Gate the assignment on respond_to? so a value that can't handle :downcase or :strip is left untouched:

self[attr] = self[attr].send(method) if self[attr].respond_to?(method)

Devise::ParameterFilter#filtered_hash_by_method_for_given_keys already takes the same approach on the params side, so this aligns the two paths.

Tests

Two tests in test/models/database_authenticatable_test.rb cover the case where an integer attribute (sign_in_count) is included in the keys list — they assert the value is preserved after valid? runs the before_validation :downcase_keys / :strip_whitespace callbacks. Both tests fail on main (the integer is replaced with nil) and pass with the fix.

Closes #5780.