Use a param file for multi_deploy merge inputs

[utkarsh-dd]

Summary:

• Spill multi_deploy's deploy-merge arguments into a Bazel multiline param file.
• Expand @argfiles in img deploy-merge before flag parsing.
• Add regression coverage for deploy-merge argfile handling.

Why:

• Large multi_deploy targets can have thousands of operations.
• Passing every deploy manifest path inline can exceed OS command-line limits.
• Bazel param files are the standard way for rules to avoid long argv failures.

Notes:

• The "deploy-merge" subcommand remains inline so the img dispatcher still works.
• This follows the existing repo pattern of using Bazel Args param files for large action inputs, as seen in layer and layer_from_binary actions.
• The public Starlark API is unchanged.
• Prebuilt binaries are not patched in this PR; releases/toolchain packaging should pick up the source change normally.