[Madrigal]

Issue #, if available: None.

Description of changes:

This adds a .gitattributes that marks generated files as linguist-generated. GitHub collapses those files by
default in PR diffs and leaves them out of language stats. It's the same approach as kubernetes/kubernetes and
aws/aws-sdk-go-v2#3636. No source changes.

The generated C++ has no "generated" header, and Linguist's header rule only applies to Go anyway, so today none of
it is collapsed. The rules follow the output paths in the generator code:

Rule	What writes it
/generated/** (except generated/smoke-tests/RunTests.cpp)	C2J/Smithy client generation (rmtree + extract), endpoint/include tests, protocol_tests_gen.py, smoke_tests_gen.py
core ClientConfigurationDefaults.*, AWSPartitions.*	CppDefaultsGenerator, CppPartitionsGenerator
tools/CI/install-test/all_services_test.cpp	install_test_gen.py
tools/code-generation/smithy/**/smithy-build.json	Smithy build.gradle.kts
api-descriptions/**, endpoints/**, defaults/partitions JSON, protocol-test models and suites	fetch_latest_models.py (verbatim upstream copy), and the Smithy protocol-test models

These stay visible on purpose: RunTests.cpp, the mixed install-test/CMakeLists.txt, Region.h, VERSION,
VersionConfig.h, CHANGELOG.md and the generator .java/.vm sources. Release automation touches them, but
they also carry hand-written changes.

Verification. I ran git check-attr linguist-generated on all 175,207 tracked files:

• 173,194 are marked, exactly the set the generator code owns (derived independently, not from the globs).
• Nothing outside that set is marked.
• Every file with a generated-style header is marked, except 7 explained cases: 4 .vm templates, the doxygen
  layout, and 2 gradlew files that Linguist already handles.

Effect on recent PRs (lines hidden by default):

PR	Lines hidden	Still shown
#3965	380 of 469	the .vm and XmlSerializer changes
#3960	96,370 of 96,633
#3920	19,168 of 19,673	generator and runner changes
#3964 (generator-only)	0	everything

For reviewers: when a PR changes the generator, expand the collapsed regenerated files to see its effect. The
copied-in models are their own block, in case you'd rather keep them visible.

No changelog fragment: this doesn't change SDK behavior (see CONTRIBUTING.md).

Check all that applies:

• Did a review by yourself.
• Added proper tests to cover this PR. (Not applicable: .gitattributes only, verified with git check-attr as above.)
• Checked if this PR is a breaking (APIs have been changed) change. (Not breaking.)
• Checked if this PR will not introduce cross-platform inconsistent behavior. (No build or runtime effect.)
• Checked if this PR would require a ReadMe/Wiki update. (No.)

Check which platforms you have built SDK on to verify the correctness of this PR.

• Linux
• Windows
• Android
• MacOS
• IOS
• Other Platforms (not applicable: no build change)

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