feat: polish model config form UI (#24047)

Polishes the AI model configuration form (add/edit model) with tighter
layout and better input affordances.

**Frontend changes:**
- Replace "Unset" with "Default" in select dropdowns to communicate
system fallback
- Show pricing fields inline instead of behind a collapsible toggle
- Use flat section dividers (`border-t`) instead of bordered fieldsets
- Move field descriptions into info-icon tooltips to fix input
misalignment
- Add InputGroup adornments: `$` prefix + `/1M` suffix on pricing,
`tokens` suffix on token fields, `%` suffix on compression threshold,
range placeholders on temperature/penalty fields
- Shorter pricing labels (Input, Output, Cache Read, Cache Write)
- Compact JSON textareas (1-row height, resizable)
- Smart grid layouts by field type (3-col provider, 4-col pricing, 3-col
advanced)
- Boolean fields render as a segmented control (Default · On · Off)
instead of a dropdown

**Backend changes:**
- Add `enum` tags to OpenAI `service_tier`
(`auto,default,flex,scale,priority`) and `reasoning_summary`
(`auto,concise,detailed`) so they render as select dropdowns instead of
free-text inputs

> 🤖 Generated by Coder Agents

scripts/modeloptionsgen/main.go

@@ -18,6 +18,7 @@ type SchemaField struct {
 	GoName      string   `json:"go_name"`
 	Type        string   `json:"type"`
 	Description string   `json:"description,omitempty"`
+	Label       string   `json:"label,omitempty"`
 	Required    bool     `json:"required"`
 	Enum        []string `json:"enum,omitempty"`
 	InputType   string   `json:"input_type"`
@@ -135,6 +136,7 @@ func extractFields(t reflect.Type, prefix string, skip map[string]bool) FieldGro
 
 		typeName := goTypeToSchemaType(f.Type)
 		description := f.Tag.Get("description")
+		label := f.Tag.Get("label")
 		enumTag := f.Tag.Get("enum")
 
 		var enumValues []string
@@ -150,6 +152,7 @@ func extractFields(t reflect.Type, prefix string, skip map[string]bool) FieldGro
 			GoName:      goFieldPath(prefix, f.Name, t, fullJSONName),
 			Type:        typeName,
 			Description: description,
+			Label:       label,
 			Required:    required,
 			Enum:        enumValues,
 			InputType:   inputType,