Implement Python SDK with PulsarSession, AgenticSession, and WebDriver APIs

[Copilot]

Python SDK implementation mirroring Kotlin interfaces (PulsarSession, AgenticSession, WebDriver) for Browser4, communicating via OpenAPI protocol.

Changes

Core Classes

• PulsarSession: Page loading (open, load, submit), URL normalization, CSS-based extraction
• AgenticSession: Extends PulsarSession with AI-powered automation (act, run, observe, summarize, agent_extract)
• WebDriver: Browser control—navigation, scrolling, element interaction, script execution, screenshots

Data Models

• WebPage, NormURL, PageSnapshot for page results
• AgentRunResult, AgentActResult, ObserveResult, ExtractionResult for agent operations
• PageEventHandlers placeholder for future event mechanism

Tests: 28 unit tests covering all major functionality

Usage

from pulsar_sdk import PulsarClient, AgenticSession

client = PulsarClient(base_url="http://localhost:8182")
client.create_session()
session = AgenticSession(client)

# PulsarSession operations
page = session.open("https://example.com")
fields = session.extract(page, {"title": "h1", "links": "a[href]"})

# Agentic operations
result = session.act("click the search button")
history = session.run("search for 'python' and click first result")

# WebDriver access
session.driver.scroll_to_bottom()
text = session.driver.select_first_text_or_null("h1")

session.close()

API Consistency

Python methods use snake_case with Kotlin-style aliases (process_trace / processTrace, final_result / finalResult).

Original prompt

实现 python SDK。Python SDK 基于 openapi 协议与 browser4 通讯。调用方式参考FusedActs。Python 中以下接口同 Kotlin 保持尽可能一致：PulsarSession/AgenticSession/WebDriver。预留事件机制（PageEventHandlers），在后面的任务中完成。

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