feat: add end-to-end tests for sqlcmd binary (fixes #641)

[dlevy-msft-sql]

Summary

Adds end-to-end tests that build the sqlcmd binary and exercise real-world
scenarios, as suggested by @shueybubbles in PR #640.

Test Cases

Test	Description
\TestE2E_Help\	Verifies --help\ flag produces expected output
\TestE2E_Version\	Verifies --version\ flag works
\TestE2E_PipedInput_NoPanic\	Regression test for #607 - piped input with -G\ flag doesn't panic
\TestE2E_PipedInput_EmptyInput\	Empty piped input doesn't cause panic
\TestE2E_InvalidFlag\	Invalid flags produce helpful error messages
\TestE2E_QueryFlag_NoServer\	-Q\ flag without server doesn't panic
\TestE2E_InputFile_NotFound\	Missing input file errors gracefully
\TestE2E_PipedInput_WithBytesBuffer_NoPanic\	GO batches in piped input work correctly

Live Connection Tests (run when SQLCMDSERVER is set)

Test	Description
\TestE2E_PipedInput_LiveConnection\	Piped input executes queries successfully
\TestE2E_QueryFlag_LiveConnection\	-Q\ flag executes queries successfully
\TestE2E_InputFile_LiveConnection\	Input file with SQL executes successfully

Implementation Details

• Binary is built once per test run using \sync.Once\ for efficiency
• Tests verify error handling and absence of panics, not successful connections
• Live connection tests skip when SQLCMDSERVER is not set
• Binary is placed in a temp directory and cleaned up after tests

Related

• Fixes Add end-to-end tests that build and run the sqlcmd binary #641
• Addresses feedback from fix: prevent nil pointer panic when piping input to sqlcmd (fixes #607) #640
• Regression tests for sqlcmd panic when run inside emacs #607

Testing

\\�ash
go test -v -run TestE2E ./cmd/modern/...
\\

All 11 tests pass locally (8 always run, 3 skip when no live connection).

[shueybubbles]

these tests should use real queries because we have live connections available in the test pipelines. they can just rely on SQLCMDSERVER being set etc

[Copilot]

Pull request overview

This PR adds comprehensive end-to-end tests for the sqlcmd binary that build and execute the actual binary to verify behavior in real-world scenarios. The tests primarily focus on verifying that error conditions (like piped input, missing files, invalid flags) are handled gracefully without panics, especially addressing the regression reported in issue #607.

Changes:

• Adds 8 e2e tests covering help, version, piped input, invalid flags, query execution, and input file handling
• Implements a build-once pattern using sync.Once for efficiency across all test cases
• Uses TestMain for cleanup of the temporary binary after tests complete

[Copilot]

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Pull request overview

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