Include upstream request headers in Foundry fork command and add unit test

[fubuloubu]

Motivation

• Ensure that when forking an upstream network, any required HTTP request headers (like auth or tracing headers) are forwarded to the forked node so upstream requests include the same context.

Description

• Retrieve upstream headers via network_manager.get_request_headers using forked_network.ecosystem.name, forked_network.upstream_network.name, and upstream_provider_name inside FoundryForkProvider.build_command.
• Append each returned header to the foundry/anvil command as --fork-header "key: value" so they are passed when starting the fork.
• Add an import for PropertyMock and a new unit test test_fork_build_command_adds_upstream_request_headers in tests/test_provider.py that verifies get_request_headers is called and --fork-header entries are included in the built command.

Testing

• Ran the new unit test pytest tests/test_provider.py::test_fork_build_command_adds_upstream_request_headers and it passed.
• Ran the provider test file pytest tests/test_provider.py to ensure no regressions in provider behavior and all tests passed.

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Codex Task