Replace fixed header buffer with dynamic ArrayList in sendHttpRequest

[Copilot]

sendHttpRequest used a fixed 4096-byte stack buffer for building HTTP request headers. Requests with long paths or Content-Type values could exhaust the buffer, returning error.NoSpaceLeft instead of sending the request.

Description

Replaces the fixed [4096]u8 + fixedBufferStream with std.ArrayList(u8) that grows on demand. The function gains an allocator: std.mem.Allocator parameter; callers pass their self.allocator.

Before:

fn sendHttpRequest(stream: anytype, ...) !void {
    var hdr_buf: [4096]u8 = undefined;
    var fbs = std.io.fixedBufferStream(&hdr_buf);
    const w = fbs.writer();
    // ...
    try stream.writeAll(fbs.getWritten());
}

After:

fn sendHttpRequest(allocator: std.mem.Allocator, stream: anytype, ...) !void {
    var hdr_buf = std.ArrayList(u8).init(allocator);
    defer hdr_buf.deinit();
    const w = hdr_buf.writer();
    // ...
    try stream.writeAll(hdr_buf.items);
}

• doRequest, imagePull, and ping updated to pass self.allocator
• Unit tests updated to pass std.testing.allocator

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring
• Performance improvement
• Test addition/modification
• CI/CD changes

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

Testing

• Unit tests

Existing sendHttpRequest unit tests pass with std.testing.allocator substituted for the removed stack buffer.

Additional Notes

Internal function signature change only — no public API impact.

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