revert(captain): undo OpenAI provider switch (wrong scope)

The OpenAI Custom Server request targets the per-account OpenAI integration
(/app/accounts/{id}/settings/integrations/openai), not the instance-level
Captain config. The CAPTAIN_OPEN_AI_PROVIDER provider-switch added in 46fe70398
was out of the intended scope and is new behavior that did not previously exist,
so it is removed. This restores the original Captain behavior (CAPTAIN_OPEN_AI_ENDPOINT
remains usable as before, no provider gating). The correct implementation goes in
config/integration/apps.yml for the per-account openai integration.

app/models/installation_config.rb

@@ -37,8 +37,6 @@ class InstallationConfig < ApplicationRecord
   before_validation :set_lock
   validates :name, presence: true
   validate :saml_sso_users_check, if: -> { name == 'ENABLE_SAML_SSO_LOGIN' }
-  validate :openai_custom_provider_check, if: -> { name == 'CAPTAIN_OPEN_AI_PROVIDER' }
-  validate :openai_custom_endpoint_check, if: -> { name == 'CAPTAIN_OPEN_AI_ENDPOINT' }
 
   # TODO: Get rid of default scope
   # https://stackoverflow.com/a/1834250/939299
@@ -73,28 +71,4 @@ class InstallationConfig < ApplicationRecord
 
     errors.add(:base, 'Cannot disable SAML SSO login while users are using SAML authentication')
   end
-
-  # Only allow provider values the platform understands ('openai' default or 'custom')
-  # and ensure the "custom" provider actually requires an endpoint.
-  def openai_custom_provider_check
-    return if %w[openai custom].include?(value.to_s)
-
-    errors.add(:base, 'CAPTAIN_OPEN_AI_PROVIDER must be "openai" or "custom"')
-  end
-
-  # Fail-closed: when the provider is "custom", CAPTAIN_OPEN_AI_ENDPOINT must be
-  # present AND a secure https URL (OpenAI-compatible base).
-  def openai_custom_endpoint_check
-    provider = InstallationConfig.find_by(name: 'CAPTAIN_OPEN_AI_PROVIDER')&.value.to_s
-    return unless provider == 'custom'
-
-    if value.blank?
-      errors.add(:base, 'CAPTAIN_OPEN_AI_ENDPOINT is required when using the custom provider')
-      return
-    end
-
-    return if value.to_s.match?(%r{\Ahttps://\S+})
-
-    errors.add(:base, 'CAPTAIN_OPEN_AI_ENDPOINT must be a valid https URL when using the custom provider')
-  end
 end