[dhcpmon] Clean up source include dependencies

[Xichen96]

Description of PR

Summary:

Make source dependencies explicit and keep public headers self-contained. The change removes unused or redundant includes, moves implementation-only dependencies into .cpp files, and adds direct includes where code previously relied on transitive imports.

Dependencies: None.

Work item tracking

• Microsoft ADO (number only): 38919307
• Child Task: 38919308

Type of change

• Bug fix
• New feature
• Refactor / cleanup
• Documentation update
• Test improvement

Approach

What is the motivation for this PR?

Several files rely on unrelated headers to expose declarations such as errno, smart pointers, fixed-width integers, and size_t. Other files retain imports they do not use. This makes compilation dependencies fragile and increases header coupling.

How did you do it?

• Added direct implementation includes for symbols used by each source file.
• Made public headers include the declarations required by their own APIs and inline templates.
• Moved smart-pointer dependencies out of util.h and into their implementation users.
• Removed unused and redundant system, SWSS, libevent, and standard-library includes.

How did you verify/test it?

• Reviewed all 19 C++ source and header files for direct symbol ownership and header self-containment.
• Focused diff review found no semantic or dependency issues.
• PR CI build 1173128 passed for amd64, arm64, and armhf; DCO passed.

Any platform specific information?

No. This is a compile-dependency cleanup with no runtime or platform-specific behavior change.

Back port request

None.

Tested branch

• 202605

Test result

PR CI passed on amd64, arm64, and armhf.

Documentation

Not applicable; there is no user-facing behavior or interface change.

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[Copilot]

Pull request overview

This PR refactors the C++ include structure to make dependencies explicit, reduce transitive-include reliance, and keep public headers self-contained—helping keep compilation dependencies stable as the codebase evolves.

Changes:

• Updated public headers to directly include standard/library headers required by their exposed APIs and inline helpers.
• Moved implementation-only dependencies (e.g., <memory>, <vector>, <errno.h>) into .cpp files where the symbols are actually used.
• Removed unused/redundant includes across multiple translation units to reduce coupling.

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/util.h	Makes the header self-contained for its inline helpers and exported APIs (adds required C/stdlib and <utility> includes; drops transitive deps like <memory>).
src/util.cpp	Adds direct implementation includes required for exceptions, smart pointers, and stdio/stdlib usage.
src/sock_mgr.h	Adds <stddef.h> so size_t usage in the header doesn’t rely on transitive includes.
src/sock_mgr.cpp	Adds direct includes (<arpa/inet.h>, <memory>) used by the implementation.
src/packet_handler.h	Switches to <string> for the public interface types and drops unused <string.h> from the header.
src/packet_handler.cpp	Adds direct includes used by the implementation (<errno.h>, <string.h>, <vector>).
src/main.cpp	Removes unused headers and adds direct includes required by the file’s usage (stdint/stdio).
src/dhcp_mon.h	Adds <stddef.h> to make the size_t API self-contained.
src/dhcp_mon.cpp	Moves implementation-only includes into the .cpp and removes unused ones.
src/dhcp_devman.h	Removes unused <stdint.h> from the header.
src/dhcp_devman.cpp	Adds <memory> to cover smart pointer declarations used in the implementation.
src/dhcp_device.cpp	Adds direct includes for errno/stdlib/socket and unordered containers used in the implementation.