[Repo Assist] perf(mcp): eliminate LINQ and ToArray() allocations in McpToolBridge

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🤖 This is an automated PR from Repo Assist.

Two small allocation wins in the MCP hot path — one per tool call dispatch, one per JSON-RPC response:

1. System.Linq removed — FirstOrDefault → foreach

HandleToolsCallAsync was using LINQ FirstOrDefault with a lambda to find the capability for a tool call. Replaced with a plain foreach + break:

// Before
var capability = caps.FirstOrDefault(c => c.CanHandle(name));

// After
INodeCapability? capability = null;
foreach (var c in caps)
{
    if (!c.CanHandle(name)) continue;
    capability = c;
    break;
}

This follows the existing pattern in the codebase of eliminating LINQ in hot paths (see previous PRs removing LINQ from SystemCapability, ExecShellWrapperParser, etc.).

2. ms.ToArray() → ms.GetBuffer() + slice in WriteResult / WriteError

MemoryStream.ToArray() allocates a new byte[] containing a copy of the stream contents. Using GetBuffer() with the actual length avoids that copy before the UTF-8 decode:

// Before
return System.Text.Encoding.UTF8.GetString(ms.ToArray());

// After
return System.Text.Encoding.UTF8.GetString(ms.GetBuffer(), 0, (int)ms.Length);

Both WriteResult and WriteError are touched. This saves one byte array allocation per JSON-RPC response.

Test Status

Suite	Result
OpenClaw.Shared.Tests (full suite)	✅ 964 passed, 20 skipped; 3 pre-existing McpHttpServerTests Linux failures unrelated to this change (tracked in PR #238)
OpenClaw.Tray.Tests	✅ 202 passed, 0 failed

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