fix: keep the Msg/Level/State header on errors sent to stderr with -r

[Raymondriter]

Fixes #794

Problem

With -r, runQuery passed only e.Message to the PrintError hook. Because the hook returns true, Formatter.AddError was skipped, and that's where the Msg N, Level N, State N, Server S, Line N header is built. Errors on stderr lost the error number and line. ODBC sqlcmd prints the same text with -r and only changes the destination.

Fix

• Move the header formatting from AddError into errorHeader(e mssql.Error). The localized format strings are unchanged, so existing translations still apply.
• Pass errorHeader(e) + e.Message to PrintError, so a host that redirects errors gets the text the default output would show.
• The PrintError signature is unchanged. I documented on the field that msg includes the header for server errors.

Tests

• New TestPrintErrorIncludesErrorHeader runs RAISERROR (N'Testing!', 11, 1) with a redirecting hook. It asserts the hook receives Msg 50000, Level 11, State 1, Server ..., Line 1 followed by the message, and that nothing reaches the default output. On main the hook receives only Testing!.

• Full suite against a local SQL Server 2022 container: the same results as main. The only failures are the environment-dependent install/ADS/editor tests.

• End to end with the repro from the issue (sqlcmd -r -b -i repro.sql), stderr now reads:

  Msg 207, Level 16, State 1, Server <host>, Line 1
  Invalid column name 'id'.
  

  This matches the output without -r.

Not addressed here: the issue's additional observation that -b stops after the first error of a batch. It's a separate behaviour change, and I can follow up in its own PR.

🤖 Generated with Claude Code

[Copilot]

🟡 Changes recommended

Redirected errors omit the mssql: prefix when -r is combined with -j.

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Pull request overview

Fixes redirected SQL Server errors so -r output retains the standard diagnostic header.

Changes:

• Centralizes localized error-header formatting.
• Includes headers in redirected error messages.
• Adds integration coverage and documents the PrintError contract.

File summaries

File	Summary
pkg/sqlcmd/sqlcmd.go	Updates redirected error handling and documentation.
pkg/sqlcmd/sqlcmd_test.go	Tests redirected error headers.
pkg/sqlcmd/format.go	Centralizes server error header formatting.

Review details

• Files reviewed: 3/3 changed files
• Comments generated: 1
• Review effort level: Lite

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[Raymondriter]

Withdrawing this for now. The root cause and fix are described above in case they're useful to anyone picking up #794.