feat: Add beta mode support to Message builder

[thibauult]

Summary

• Add support for the beta attribute on the <messageML> tag, as introduced in messageml-utils PR #421
• This enables experimental features like <sym-ai-context> for Symphony AI integration
• When beta=true, messages are wrapped with <messageML beta="true"> instead of <messageML>

Test plan

• checkBetaModeNotAddedByDefault() - verifies default behavior has no beta attribute
• checkBetaModeAddedWhenTrue() - verifies beta="true" is added when set
• checkBetaModeNotAddedWhenFalse() - verifies no beta attribute when explicitly set to false
• checkBetaTrueWithPreWrappedContentThrowsException() - verifies exception when beta=true but content already wrapped without beta
• checkBetaTrueWithPreWrappedBetaContentAllowed() - verifies content with <messageML beta="true"> is accepted

Usage Example

Message message = Message.builder()
    .content("<sym-ai-context>...</sym-ai-context>")
    .beta(true)
    .build();
// Results in: <messageML beta="true"><sym-ai-context>...</sym-ai-context></messageML>