Honor NO_COLOR env var in TerminalOutputDevice (#8825)

[Evangelink]

Fixes #8825.

Adds support for the de-facto cross-tool NO_COLOR environment variable in TerminalOutputDevice. When NO_COLOR is set with any non-empty value, MTP suppresses ANSI escape sequences in terminal output, matching the behavior of the existing --no-ansi flag and LLM environment detection.

Precedence (unchanged ordering, NO_COLOR slotted in)

1. --ansi on -> ForceAnsi (wins over everything, including NO_COLOR)
2. --ansi off -> NoAnsi
3. --no-ansi OR NO_COLOR set OR LLM env detected -> NoAnsi
4. CI detected -> SimpleAnsi
5. default -> AnsiIfPossible

Trade-off note

The NO_COLOR spec strictly only forbids ANSI color codes. This PR maps NO_COLOR to MTP's NoAnsi mode, which also disables cursor movement / live progress, matching what --no-ansi does. This is the user-intuitive behavior most CLIs converge on in practice, and keeps the change tight. If a future user wants "live progress without color" specifically, a follow-up could introduce a SimpleAnsiNoColor mode without breaking this contract.

Tests

Three new acceptance tests in AnsiOptionTests:

• NoColorEnvVar_NonEmpty_DisablesAnsiOutput -- in a (mocked) CI env with --ansi auto, setting NO_COLOR=1 produces output with no ESC.
• NoColorEnvVar_Empty_IsIgnored -- empty NO_COLOR is a no-op per spec; SimpleAnsi ESC still emitted.
• NoColorEnvVar_OverriddenByExplicitAnsiOn -- --ansi on overrides NO_COLOR.

NO_COLOR was added to WellKnownEnvironmentVariables.ToSkipEnvironmentVariables so an ambient developer/CI value cannot leak into other acceptance tests.

---

Part of the agent/LLM-efficient test output effort tracked in #8824.

[Copilot]

Pull request overview

Adds first-class support for the cross-tool NO_COLOR environment variable to Microsoft.Testing.Platform’s terminal output, ensuring ANSI escape sequences are suppressed when NO_COLOR is set (unless explicitly overridden by --ansi on). This aligns MTP behavior with common CLI ecosystem conventions while preserving the existing precedence rules.

Changes:

• Update TerminalOutputDevice ANSI-mode resolution to treat non-empty NO_COLOR as AnsiMode.NoAnsi (below explicit --ansi on|off, alongside --no-ansi and LLM detection).
• Add acceptance coverage for NO_COLOR (non-empty disables ANSI, empty is ignored, --ansi on overrides).
• Prevent ambient NO_COLOR from leaking into other acceptance tests by filtering it out in test infrastructure environment propagation.

Show a summary per file

File	Description
src/Platform/Microsoft.Testing.Platform/OutputDevice/TerminalOutputDevice.cs	Honors non-empty NO_COLOR by selecting AnsiMode.NoAnsi unless explicitly overridden by --ansi on.
test/IntegrationTests/Microsoft.Testing.Platform.Acceptance.IntegrationTests/AnsiOptionTests.cs	Adds acceptance tests validating NO_COLOR behavior and precedence vs --ansi on / CI auto-detection.
test/Utilities/Microsoft.Testing.TestInfrastructure/WellKnownEnvironmentVariables.cs	Adds NO_COLOR to the “skip/clear ambient env vars” list to avoid test flakiness from developer/CI shells.

Copilot's findings

• Files reviewed: 3/3 changed files
• Comments generated: 0