refactor: rewrite model-context to use js-yaml, add context_length to provider form (#177)

* fix: context-length API returns 200K instead of actual model context

Two bugs cause the /api/hermes/sessions/context-length endpoint to
always return DEFAULT_CONTEXT_LENGTH (200K):

1. getModelContextLength ignores config.yaml model.context_length
   The function only checks models_dev_cache.json (which doesn't
   exist in default installations) and falls back to the hardcoded
   200K default, completely ignoring the user's explicit
   model.context_length setting in config.yaml.

2. getDefaultModel regex fails when api_key/base_url come before default
   The regex /^model:\s*\n\s+default:\s*(.+)$/m assumes 'default' is
   the first child key under 'model:', but when api_key or base_url
   appear first in the YAML, the match fails. This causes
   getModelContextLength to short-circuit to DEFAULT_CONTEXT_LENGTH
   before even reaching the cache lookup.

Fix:
- Add getDefaultModelRobust() that extracts the entire model: block
  first, then searches for default: within it
- Add getConfigContextLength() that reads model.context_length from
  config.yaml as a fallback (matching hermes-agent priority)
- Update getModelContextLength() resolution order:
  1. models_dev_cache.json (existing)
  2. config.yaml model.context_length (new)
  3. DEFAULT_CONTEXT_LENGTH (existing fallback)

Closes #169

* refactor: rewrite model-context to use js-yaml, add context_length to provider form

- Replace fragile regex-based YAML parsing with js-yaml for reliable config.yaml reads
- Fix context_length resolution priority: config.yaml override > custom_providers > models_dev_cache > 200K default
- Add context_length input field when adding custom providers in ProviderFormModal
- Backend: persist context_length to custom_providers models.<model>.context_length in config.yaml
- Add i18n keys (contextLength, contextLengthPlaceholder) to all 8 locales

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix: use NInputNumber instead of NInput type=number for context_length

NInput does not support type="number" in Naive UI.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: devilardis <53129661@qq.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

packages/client/src/components/hermes/models/ProviderFormModal.vue

@@ -1,6 +1,6 @@
 <script setup lang="ts">
 import { ref, watch, computed, onMounted } from 'vue'
-import { NModal, NForm, NFormItem, NInput, NButton, NSelect, useMessage } from 'naive-ui'
+import { NModal, NForm, NFormItem, NInput, NInputNumber, NButton, NSelect, useMessage } from 'naive-ui'
 import { useModelsStore } from '@/stores/hermes/models'
 import { useI18n } from 'vue-i18n'
 import CodexLoginModal from './CodexLoginModal.vue'
@@ -29,6 +29,7 @@ const formData = ref({
   base_url: '',
   api_key: '',
   model: '',
+  context_length: null as number | null,
 })
 
 const modelOptions = ref<Array<{ label: string; value: string }>>([])
@@ -75,7 +76,7 @@ watch(() => formData.value.base_url, (url) => {
 
 watch(providerType, () => {
   modelOptions.value = []
-  formData.value = { name: '', base_url: '', api_key: '', model: '' }
+  formData.value = { name: '', base_url: '', api_key: '', model: '', context_length: null }
   selectedPreset.value = null
 })
 
@@ -154,11 +155,13 @@ async function handleSave() {
       ? selectedPreset.value
       : null
 
+    const contextLength = formData.value.context_length ?? undefined
     await modelsStore.addProvider({
       name: formData.value.name.trim(),
       base_url: formData.value.base_url.trim(),
       api_key: formData.value.api_key.trim(),
       model: formData.value.model,
+      context_length: contextLength,
       providerKey,
     })
     message.success(t('models.providerAdded'))
@@ -270,6 +273,16 @@ function handleClose() {
           </NButton>
         </div>
       </NFormItem>
+
+      <NFormItem v-if="providerType === 'custom'" :label="t('models.contextLength')">
+        <NInputNumber
+          v-model:value="formData.context_length as number | null"
+          :placeholder="t('models.contextLengthPlaceholder')"
+          :min="0"
+          clearable
+          style="width: 100%"
+        />
+      </NFormItem>
     </NForm>
 
     <template #footer>