chore: Add license header enforcement to all source files

[Copilot]

Every source file (.rs, .ts, .js, .cs, .h, .proto) must carry the Microsoft copyright header. No enforcement existed—this adds the check to cargo xtask check and mass-adds the header to all 157 existing files.

New: xtask/src/license_headers.rs

• Iterative tree walker that checks every source file for the two-line header
• check() fails with a list of non-compliant files; fix() prepends missing headers
• Skips target/, node_modules/, dist/, obj/, bin/, .git/ and auto-generated files (webui_ffi.h)
• 7 unit tests including a workspace-wide regression guard

cargo xtask check integration

• LICENSE_HEADERS runs as the first step in the pipeline, before fmt
• Standalone command: cargo xtask license-headers [--fix]

// Copyright (c) Microsoft Corporation.
// Licensed under the MIT license.

Copilot instructions update

• Added "License header" section under code organization
• Updated pipeline description, commands table, and acceptance checklist

Original prompt

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This section details on the original issue you should resolve

<issue_title>Every file must have a license header</issue_title>
<issue_description>File Header. Include the following header in each source file.

// Copyright (c) Microsoft Corporation.
// Licensed under the MIT license.

And make sure the linter (cargo deny) verifies this in CI as well for example cargo xtask check should fail becuase it includes deny check.

We need to enforce .rs, cs, .h, .ts and any source file to have this file header</issue_description>

<agent_instructions>We need a recommended way to always have these file header, so include it in the custom instructions as well to enforce it, and cargo deny, but make sure other ones have it. You can exclude .html/.css/.json/.yml/.xml from this check.

and do a mass adding this header to all files. You can skip .html/.css/.json/.yml files.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Every file must have a license header #111

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