[Dev UI] refactor: decompose convertPartsToDomNodes in MessageRenderingService

[MichaelDoyle]

Problem

MessageRenderingService.convertPartsToDomNodes in src/app/services/message-rendering.service.ts has grown to ~140 lines (~30% of the service) and combines multiple competing responsibilities in a single procedural loop:

1. 12-branch if / else if dispatch ladder: Dispatches across text, reasoning, 5 media variants, toolRequest, resource, custom, a2ui, and fallback data.
2. Imperative loop mutation (i += consumed): Inside the render loop, combineAdjacentStringParts mutates the loop index i mid-flight to coalesce streaming chunks. This couples stream normalization with DOM construction and makes index tracking fragile alongside index-dependent lookups (such as A2UI surface anchoring and leftovers mapping).
3. Disproportionate complexity: Complex protocol logic (such as A2UI surface card anchoring, container reshaping, and leftovers fallback) lives inline inside the loop body.

Proposed Solution

Adopt a phased pipeline similar to the strategy pattern used for trace previews in src/app/components/json-viewer/render-types:

1. Upstream Normalization Pass:
   • Introduce a pure, framework-agnostic normalizeParts(parts: Part[]): Part[] helper that coalesces adjacent string parts (text/reasoning lacking metadata) upfront.
   • Eliminates lookaheads, consumed counters, and i += consumed index mutations during rendering.
2. Modular Part Renderers / Strategies:
   • Extract individual branch logic into dedicated renderers/helpers (e.g., A2uiPartRenderer, MediaPartRenderer, TextPartRenderer).
   • Replace the 12-branch ladder with an ordered registry of handlers (canRender(part) / render(part, context)).

Benefits

• Testability: Normalization and individual part handlers can be unit-tested in isolation without full Angular component/DOM harnesses.
• Maintainability & OCP: Adding support for future part types will no longer require expanding the central render loop.
• Safer Iteration: Removes manual loop pointer manipulation and cleanly decouples stream parsing from DOM generation.