Fix dab validate error messaging when logger is not available

[aaronburtle]

Why make this change?

Closes #3268

What is this change?

When dab validate or dab start encounters a config parsing error (e.g. missing entities/autoentities), the CLI previously dumped the full exception message and stack trace to stderr. This made the output noisy and unhelpful. After this change, only a clean, descriptive validation message is shown.

The key design changes:

• TryParseConfig is now a pure function that returns an out string? parseError message on failure instead of writing to Console.Error or ILogger internally. Error reporting is the caller's responsibility.
• FileSystemRuntimeConfigLoader.TryLoadConfig writes the parseError to Console.Error (because config is parsed before the DI container and logger are available, so the log buffer would never be flushed on a parse failure) and sets an instance-scoped IsParseErrorEmitted flag so CLI callers (ConfigGenerator) can avoid logging duplicate messages.
• ConfigGenerator.IsConfigValid now has an explicit early-return path when config parsing fails (via runtimeConfigProvider.TryGetConfig), using IsParseErrorEmitted to suppress duplicate output.
• ValidateOptions.Handler uses LogError("Config is invalid.").
• Comments referencing which method emits the error to Console.Error corrected to TryLoadConfig (not TryParseConfig).

How was this tested?

• ValidateConfigTests.cs — Added TestValidateConfigWithNoEntitiesProducesCleanError (new test verifying clean error message, no stack traces).
• EnvironmentTests.cs — Updated FailureToStartEngineWhenEnvVarNamedWrong to match the new single-line clean stderr format.
• EndToEndTests.cs — Simplified assertions in TestExitOfRuntimeEngineWithInvalidConfig (this test is Ignored but updated for consistency).
• RuntimeConfigLoaderTests.cs — Updated FailLoadMultiDataSourceConfigDuplicateEntities to assert loader.IsParseErrorEmitted is true.

[Copilot]

Pull request overview

This PR improves the CLI experience for dab validate / dab start when config parsing fails by surfacing a clean, user-focused error message (instead of dumping exception details/stack traces), and updates tests to match the new output.

Changes:

• Add a mechanism (RuntimeConfigLoader.LastParseError) to capture the last config parse error message for callers (CLI) to display cleanly.
• Update CLI config loading/validation paths to log the captured parse error and (for validate) suppress raw Console.Error output during config loading.
• Update CLI and service/CLI tests to assert the new single-line/clean error behavior.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
src/Config/RuntimeConfigLoader.cs	Captures last parse error and changes exception handling output/logging behavior.
src/Cli/ConfigGenerator.cs	Uses LastParseError when config load fails; suppresses Console.Error during validation config load.
src/Cli/Commands/ValidateOptions.cs	Changes “Config is invalid” message severity/output format.
src/Cli.Tests/ValidateConfigTests.cs	Adds regression coverage ensuring clean error messages (no stack traces) for missing entities/autoentities.
src/Cli.Tests/EnvironmentTests.cs	Updates stderr assertions for new clean error format.
src/Cli.Tests/EndToEndTests.cs	Removes outdated assertions in ignored test to align with new behavior.
src/Service.Tests/Configuration/RuntimeConfigLoaderTests.cs	Updates assertions to validate clean error output without stack traces.

[aaronburtle]

@copilot can you do another PR review please.

[souvikghosh04]

posting comments so far

[Aniruddh25]

Waiting on merge with #3318

[souvikghosh04]

LGTM. thanks for addressing the comments