feat: expose resumableStreams and openConnectionOnStartup on McpClientBuilder

[wkflash]

Summary

Expose resumableStreams and openConnectionOnStartup on McpClientBuilder for Streamable HTTP transport, with
bidirectional call-order safety.

Closes #2003.

Problem

When connecting to a Streamable HTTP MCP server that disables SSE and only accepts POST, tool calls hang indefinitely. The
underlying MCP Java SDK (HttpClientStreamableHttpTransport.Builder) already supports these options, but AgentScope's
builder did not expose them.

Solution

Two new fluent methods + tools.json config support:

Java API

McpClientBuilder.create("example")
    .streamableHttpTransport("https://sse-disabled.example.com/mcp")
    .resumableStreams(false)
    .openConnectionOnStartup(false)
    .buildSync();

tools.json

{
  "mcpServers": {
    "my-server": {
      "transport": "http",
      "url": "https://example.com/mcp",
      "resumableStreams": false,
      "openConnectionOnStartup": false
    }
  }
}

Bidirectional safety

Unlike the approach that only writes to the config object, this PR stores values in both a builder-level field AND syncs to
the config if already present. Both call orders work:

// order 1: transport first → instanceOf check writes to config

// order 2: options first → builder field stored, applied when transport is created
.resumableStreams(false).streamableHttpTransport(url)

Tests

- resumableStreams / openConnectionOnStartup after streamableHttpTransport → stored correctly
- resumableStreams / openConnectionOnStartup before streamableHttpTransport → stored correctly
- Called on SSE transport → silently ignored, type unchanged
- Called before any transport → no NPE
- Both options combined
- JSON deserialization of resumableStreams / openConnectionOnStartup

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

❌ Patch coverage is 76.47059% with 8 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
.../io/agentscope/core/tool/mcp/McpClientBuilder.java	83.33%	2 Missing and 2 partials ⚠️
...tscope/harness/agent/tools/McpServerRegistrar.java	0.00%	4 Missing ⚠️

📢 Thoughts on this report? Let us know!

[jujn]

McpClientBuilder.create("mcp")
.streamableHttpTransport(url)
.resumableStreams(false)
.openConnectionOnStartup(false)

In this order, resumableStreams(false) and openConnectionOnStartup(false) are ignored because they update only the builder-level fields, not the already-created StreamableHttpTransportConfig, leaving the config fields null and thus falling back to SDK defaults.
Recommend the following:

public McpClientBuilder resumableStreams(boolean value) {
    this.resumableStreams = value;
    if (transportConfig instanceof StreamableHttpTransportConfig config) {
        config.resumableStreams = value;
    }
    return this;
}

public McpClientBuilder openConnectionOnStartup(boolean value) {
    this.openConnectionOnStartup = value;
    if (transportConfig instanceof StreamableHttpTransportConfig config) {
        config.openConnectionOnStartup = value;
    }
    return this;
}

[jujn]

Additionally, it is recommended to supplement corresponding tests