feat(channel): 跨平台 IM 接入抽象契约（UnifiedMessage/PlatformAdapter/Capabilities/Actor 身份）

internal/channel/adapter/capabilities.go

@@ -0,0 +1,61 @@
+package adapter
+
+// Capabilities declares what a concrete platform adapter supports, so the
+// runtime can degrade gracefully and the channel layer can enforce limits.
+type Capabilities struct {
+	// MaxMessageLength is the maximum message length the platform accepts.
+	// Zero means "derive a sensible default".
+	MaxMessageLength int
+	// Markdown reports whether the platform renders markdown in outgoing text.
+	Markdown bool
+	// Image reports whether the platform supports sending images.
+	Image bool
+	// Voice reports whether the platform supports sending voice/audio.
+	Voice bool
+	// Reaction reports whether the platform supports reactions.
+	Reaction bool
+	// TypingHint reports whether the platform supports typing indicators.
+	TypingHint bool
+	// RecommendBurstMax is the recommended maximum number of messages in one
+	// burst. Zero means "derive a sensible default".
+	RecommendBurstMax int
+	// Proactive reports whether the platform allows push without a recent
+	// inbound message (e.g. weixin-ilink has a 24h window; OneBot needs rate
+	// limiting). See design/08 section 3.
+	Proactive bool
+}
+
+const (
+	// defaultMaxMessageLength is used when an adapter declares no limit.
+	defaultMaxMessageLength = 4000
+	// defaultRecommendBurstMax avoids flooding a channel by default.
+	defaultRecommendBurstMax = 1
+)
+
+// Normalize fills zero-valued numeric fields with sensible defaults so a
+// partially declared Capabilities is still usable. Boolean fields are left
+// untouched because an unset bool already means "not supported".
+func (c Capabilities) Normalize() Capabilities {
+	if c.MaxMessageLength <= 0 {
+		c.MaxMessageLength = defaultMaxMessageLength
+	}
+	if c.RecommendBurstMax <= 0 {
+		c.RecommendBurstMax = defaultRecommendBurstMax
+	}
+	return c
+}
+
+// DefaultCapabilities returns a conservative baseline for adapters that only
+// speak plain text and basic images, leaving richer features off by default.
+func DefaultCapabilities() Capabilities {
+	return Capabilities{
+		MaxMessageLength:  defaultMaxMessageLength,
+		Markdown:          true,
+		Image:             true,
+		Voice:             false,
+		Reaction:          false,
+		TypingHint:        false,
+		RecommendBurstMax: defaultRecommendBurstMax,
+		Proactive:         false,
+	}
+}