refactor(anthropic): merge consecutive same-role messages in provider conversion layer

[ahmedhesham6]

Description

Move the fix for Anthropic's alternating-role requirement from a pre-API sanitization step in the CLI layer (sanitize_tool_results) into the Anthropic provider's conversion pipeline (build_messages_with_caching).

When multiple tool results are converted individually, each Role::Tool message becomes a separate role="user" Anthropic message, violating Anthropic's strict alternating user/assistant requirement. The new merge_consecutive_messages() function combines these into a single user message with all tool_result content blocks — fixing the problem at the exact layer where it originates.

Related Issues

N/A — proactive refactor to simplify the defense-in-depth strategy.

Changes Made

• libs/ai/src/providers/anthropic/convert.rs: Add merge_consecutive_messages() as Phase 2 of build_messages_with_caching(). Helper functions content_to_blocks() and merge_content() handle content type normalization and merging. 10 new tests covering: consecutive user/assistant messages, tool result merging, mixed string/blocks content, cache control preservation, empty/single messages, and full conversation flows with multiple tool results.
• libs/ai/src/providers/anthropic/types.rs: Add Default impl for AnthropicMessageContent to enable idiomatic std::mem::take.
• cli/src/commands/agent/run/mode_interactive.rs: Remove sanitize_tool_results() (dedup + orphan removal) and its pre-API call site. This was a ChatMessage-level workaround now replaced by the provider-level merge. Retain has_pending_tool_calls() and get_unresolved_tool_call_ids() as defense-in-depth guards. Refactor remaining tests to use shared helpers (test_tool_call, assistant_with_tool_calls, tool_message).

Why this is safe

• Role alternation: Handled by the new merge_consecutive_messages() in the Anthropic provider — the only provider that requires it.
• Dedup tool results: Handled by TaskBoardContextManager::dedup_tool_results() (other context managers flatten to text, making duplication irrelevant).
• Pending tool calls: Still guarded by has_pending_tool_calls() before every API call.
• Orphaned tool_use blocks: Still guarded by get_unresolved_tool_call_ids() in the user message handler.

Testing

• All tests pass (cargo test --workspace — 172 bin + all lib tests)
• cargo clippy --all-targets — zero warnings
• cargo fmt --check — clean
• Tested on macOS

Breaking Changes

None — internal refactor only, no public API changes.