Add end-to-end tests that build and run the sqlcmd binary

[dlevy-msft-sql]

Background

As noted by @shueybubbles in PR #640:

feels like we need some tests that build the app in /cmd/modern and run it. This case is a good one to start with.

Proposal

Add e2e tests that actually build the sqlcmd binary and exercise real-world scenarios. The piped input fix from #640 would be a good first test case.

Suggested test cases

1. Piped input - Verify that piping SQL to sqlcmd works without panicking (regression test for sqlcmd panic when run inside emacs #607)
2. Basic query execution - Verify -Q\ flag works correctly
3. Input file execution - Verify -i\ flag works correctly
4. Help output - Verify --help\ returns expected output

Implementation notes

• Tests would live in \cmd/modern/e2e_test.go\ or similar
• Use \go build\ to compile the binary, then \�xec.Command\ to run it
• May need build tags to skip in CI if no SQL Server is available for connection tests
• Non-connection tests (like help, version, piped input that fails gracefully) can run anywhere

Related

• fix: prevent nil pointer panic when piping input to sqlcmd (fixes #607) #640 - fix: prevent nil pointer panic when piping input to sqlcmd (fixes sqlcmd panic when run inside emacs #607)
• sqlcmd panic when run inside emacs #607 - sqlcmd panic when run inside emacs