feat(messages): enforce 2000 character message length limit

[ptone]

Summary

• Adds MaxMessageLength = 2000 constant in pkg/messages/types.go
• Validates message content length in StructuredMessage.Validate() before dispatch to any integration
• Adds early validation in handleAgentOutboundMessage handler for agent outbound messages
• Returns a clear error with current length and suggestion to split into multiple messages

Test plan

• Added test for message exceeding character limit (2001 chars)
• Added test for message at exact character limit (2000 chars)
• All existing tests pass
• go build ./... passes

[gemini-code-assist]

Code Review

This pull request introduces a 2000-character limit (MaxMessageLength) for messages, implementing validation checks in both the agent messaging handler and the structured message validation logic, along with corresponding unit tests. The review feedback highlights two main areas for improvement: first, using utf8.RuneCountInString instead of len([]rune(...)) to avoid inefficient memory allocations; second, conditionally applying the character limit validation only to outbound agent messages rather than unconditionally to all messages, which could otherwise block valid inbound user messages.

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[gemini-code-assist]

The comment for MaxMessageLength on line 28 states:

// Maximum length of the Msg field in characters for outbound messages.

However, StructuredMessage.Validate() enforces this 2000-character limit unconditionally on all messages. This includes inbound user-to-agent messages (e.g., instructions from user:alice to agent:backend-dev). Since user prompts, code snippets, or logs can easily exceed 2000 characters, this unconditional limit will block valid inbound messages.

If the limit is indeed only intended for outbound messages (agent-to-user or agent-to-agent), we should conditionally apply this check in Validate(), for example by checking if the sender is an agent using SenderPrefix(m.Sender) == "agent".

Additionally, converting the string to []rune twice (once for the check and once for the error message) is inefficient and causes redundant memory allocations. We should use utf8.RuneCountInString(m.Msg) and store the result in a variable.

Note: You will need to import "unicode/utf8" in this file. Also, since validMsg() in types_test.go uses Sender: "user:alice", you will need to update the test cases ("msg exceeds character limit" and "msg at character limit") to set m.Sender = "agent:some-agent" so that the validation is triggered during the tests.

	if SenderPrefix(m.Sender) == "agent" {
		if msgLen := utf8.RuneCountInString(m.Msg); msgLen > MaxMessageLength {
			return fmt.Errorf("message exceeds %d character limit (current: %d chars). Consider splitting into multiple messages using multiple scion message invocations", MaxMessageLength, msgLen)
		}
	}

[gemini-code-assist]

Using len([]rune(req.Msg)) to count characters in Go allocates a new slice of runes, which is inefficient and causes unnecessary memory allocations and garbage collection pressure.

Instead, use utf8.RuneCountInString(req.Msg) from the unicode/utf8 package, which counts the runes in-place with $O(1)$ memory complexity.

Note: You will need to import "unicode/utf8" in this file.

Suggested change

	if msgLen := len([]rune(req.Msg)); msgLen > messages.MaxMessageLength {
	ValidationError(w, fmt.Sprintf("message exceeds %d character limit (current: %d chars). Consider splitting into multiple messages using multiple scion message invocations", messages.MaxMessageLength, msgLen), nil)
	return
	}
	if msgLen := utf8.RuneCountInString(req.Msg); msgLen > messages.MaxMessageLength {
	ValidationError(w, fmt.Sprintf("message exceeds %d character limit (current: %d chars). Consider splitting into multiple messages using multiple scion message invocations", messages.MaxMessageLength, msgLen), nil)
	return
	}