Make the analysis of generated code opt-in

[meziantou]

Fix #1424

Follow-up to #1423, which added MEZIANTOU_ANALYZER_GENERATED_CODE as an opt-out. This flips it to an opt-in: the rules now skip generated code unless the variable is set.

What changed

AnalysisContextExtensions.GetAdditionalFlags returns Analyze | ReportDiagnostics only when the variable is 1 or true (trimmed, case-insensitive), and None for anything else, including the variable being unset.

Each rule's own flags remain the minimum, so nothing changes for the rules that pass more than None:

Flags passed by the rule	Rules	Behavior without the variable
None	145	Skip generated code (was: analyze and let report_generated_code decide)
Analyze	6	Analyze it, as they need the whole compilation to be correct (e.g. MA0053)
Analyze | ReportDiagnostics	23	Analyze it and report in it, as the generated file is the subject of the rule (e.g. the Blazor rules)

Why

Analyzing generated code costs build time, and a project where nothing reports in generated code pays it for nothing, which is the vast majority of them: you cannot fix code you do not own. Making it opt-in means only the projects that ask for it pay the cost.

Notes for reviewers

Two behavior changes are worth knowing, both covered by tests:

• report_generated_code = true is no longer sufficient on its own for the 145 rules that skip generated code, as it only decides what a rule reports among the code it analyzes. It now needs the environment variable too. Setting it to false still always works, as a rule can only report what it is allowed to report.
• A rule that skips generated code never sees it, so Roslyn's own detection applies instead, which also treats the symbols marked [GeneratedCode] or [DebuggerNonUserCode] as generated. A symbol marked with one of those attributes in a hand written file is therefore skipped by default, where previously only the file of the diagnostic mattered.

Also updated: the BannedSymbols.txt message, the XML documentation of ConfigureAnalysisOfGeneratedCode and GeneratedCodeReporting, docs/generated-code.md, the README section, and the Generated code guidance in AGENTS.md.

Tests

GeneratedCodeAnalysisTests simulates the variable with an OptedIn flags constant, as the rules read it once per process. Added coverage for the two new defaults (a rule configured with report_generated_code but not opted in reports nothing, and a rule opted in without configuration reports nothing) and for both sides of the [GeneratedCode] case. AnalysisContextExtensionsTests was flipped to opted-in/not-opted-in values.

dotnet test passes on the five Roslyn versions: 19808 tests, 0 failures. dotnet run --project src/DocumentationGenerator exits 0 with no markdown change.