fix: prevent consecutive same-role messages in ephemeral injections

[ramparte]

Problem

The orchestrator blindly appends ephemeral injections as new messages with whatever role the hook requests. When a hook uses context_injection_role="user" and the last message is already user role, this creates consecutive user messages that violate role alternation and confuse models about message attribution.

The loop-limit reminder had the same bug - it always appended a hardcoded user role message without checking what the last message was.

Fix

Structural prevention across all injection paths: before appending any ephemeral injection, check if the last message has the same role. If so, merge the content into the existing message with a \n\n separator. This prevents ANY hook from accidentally creating role alternation violations, regardless of its configuration.

Paths fixed:

1. provider:request default injection path - merge if same role
2. provider:request append_to_last_tool_result fallback (when last msg isn't tool) - merge if same role
3. tool:post pending injection default path - merge if same role
4. tool:post pending injection fallback (when last msg isn't tool) - merge if same role
5. Loop-limit reminder - merge into last user message instead of always appending new one

Related

Companion PR in amplifier-module-hooks-status-context fixes the hook itself to use append_to_last_tool_result=True.

Testing

• Existing tests have a pre-existing import error (amplifier_core.llm_errors not yet published) unrelated to this change
• Both files pass syntax validation
• Changes are purely defensive guards around existing append logic

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