fix(merge): reorder branches so streaming deltas always concatenate

[cpsievert]

Closes #435

Summary

merge_dicts() had an equality check (left_v == right_v → continue) before the str/dict/list branches. When a streamed text delta happened to equal the full accumulation so far (e.g. "\n" + "\n"), the second delta was silently dropped — both at the string-leaf level and at the container level (an equal dict skipped recursion entirely).

The fix reorders the branches to match langchain's original ordering (from which this code was adapted). Strings always concatenate, containers always recurse, and the equality check now only applies to remaining scalar types (int, float, bool).

All providers that stream via merge_dicts() are affected: ChatOllama(), ChatGroq(), ChatDeepSeek(), ChatOpenRouter(), ChatDatabricks(), ChatGoogle(), and ChatOpenAICompletions().

Verification

from openai.types.chat import ChatCompletionChunk
from chatlas._provider_openai_completions import OpenAICompletionsProvider

provider = OpenAICompletionsProvider(api_key="unused", model="gpt-4o-mini")

def chunk(text, last=False):
    return ChatCompletionChunk.model_validate({
        "id": "chatcmpl-1", "object": "chat.completion.chunk", "created": 1,
        "model": "gpt-4o-mini",
        "choices": [{"index": 0, "delta": {"role": "assistant", "content": text},
                     "finish_reason": "stop" if last else None}],
    })

deltas = ["\n", "\n", "Here is the answer."]

completion = None
for i, d in enumerate(deltas):
    completion = provider.stream_merge_chunks(
        completion, chunk(d, last=i == len(deltas) - 1)
    )
turn = provider.stream_turn(completion, has_data_model=False)

# Before: '\nHere is the answer.'
# After:  '\n\nHere is the answer.'
assert turn.text == "\n\nHere is the answer."