Fix startup crashes without ANSI terminal and in invariant globalization mode

[Mike Krüger (mkrueger)]

Problem

Two startup crashes in non-standard environments such as containers, headless hosts, and processes without a real console:

1. No ANSI terminal. When RadLine cannot create its line editor, it throws NotSupportedException: Terminal does not support ANSI. CreateLineEditor caught it and returned new LineEditor() as a fallback, but that constructor throws the same exception from inside the catch. The shell crashed, and the intended PromptFallback() was never reached. With --mcp, the crash also disposed the MCP host without stopping it, which logged a misleading "MCP server failed to start: Cannot access a disposed object". Even if the fallback had been reached, EOF on stdin (Console.ReadLine() returning null) was treated like a cancelled line, so the loop would have spun forever.
2. Globalization-invariant mode. With DOTNET_SYSTEM_GLOBALIZATION_INVARIANT=1, which is common in Linux containers without ICU, MessageService created Fluent.Net.MessageContext("en"). That throws CultureNotFoundException in invariant mode, so every command, including --help, crashed in the type initializer.

Changes

Non-ANSI fallback (Handle non-ANSI shell fallback)

• Editor creation failure returns null and is cached, so the error is printed once and RunAsync uses PromptFallback().
• In fallback mode, EOF on stdin ends the interactive loop. The RadLine path keeps its Ctrl+C semantics.

Invariant globalization (Handle invariant globalization for messages)

• MessageService.CreateMessageContext falls back to the invariant locale when the requested culture cannot be created, while still loading the bundled English catalog.
• README.md notes support for invariant mode.

Tests

• NonAnsiFallbackTests: with a non-ANSI console and redirected stdin, RunAsync executes the input and exits at EOF, and editor creation failure is cached.
• MessageServiceCultureTests: the locale fallback when culture creation throws.
• ShellProcessTests.InvariantGlobalization_ExecuteCommand_LoadsEnglishCatalog: launches the shell process with DOTNET_SYSTEM_GLOBALIZATION_INVARIANT=1.

[Copilot]

Copilot review overview

🟡 Changes recommended

The invariant context breaks singular pluralization, and editor initialization has a publication race.

Review effort: Balanced
Findings: 1 · 1

Open (2)

• Invariant locale forces plural selection to always use other · New
• Race in line editor failure cache exposes in-progress initialization · New

What changed in this PR

Fixes startup failures in non-ANSI terminals and globalization-invariant environments.

Changes:

• Adds cached line-editor fallback with EOF handling.
• Falls back to an invariant message context.
• Adds documentation and regression tests.

File	Description
README.md	Documents invariant-mode support.
MessageService.cs	Adds message-context culture fallback.
ShellInterpreter.cs	Adds non-ANSI fallback and EOF exit.
NonAnsiFallbackTests.cs	Tests editor failure and EOF behavior.
MessageServiceCultureTests.cs	Tests invariant context fallback.
ShellProcessTests.cs	Tests invariant-mode startup.

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[github-code-quality]

Code Coverage Overview

Languages: C#

C# / code-coverage/dotnet

The overall line coverage in commit ab7daf9 in the dev/mkrueger/fix-sta... branch remains at 64%, unchanged from commit 345ddb8 in the main branch.

Show a line coverage summary of the most impacted files.

File	main 345ddb8	dev/mkrueger/fix-sta... ab7daf9	+/-
D:\a\CosmosDBSh...lInterpreter.cs	69%	70%	+1%
D:\a\CosmosDBSh....Highlighter.cs	81%	83%	+2%
D:\a\CosmosDBSh...ssageService.cs	76%	78%	+2%
D:\a\CosmosDBSh...sShellPrompt.cs	53%	85%	+32%