Docs: fix Context Settings example and RST formatting

[ShaileshRawat1403]

PR Title

Docs: fix Context Settings example and RST formatting in testing guide

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PR Description

What this PR changes

This PR improves the “Testing Click Applications” documentation by:

• Converting mixed Markdown-style formatting into valid, consistent reStructuredText
• Fixing Python indentation and code block declarations so examples render correctly
• Correcting the Context Settings example to avoid misleading assertions
• Clarifying how terminal_width affects formatting without altering command behavior

No APIs, behavior, or teaching intent are changed. This is a documentation-only fix.

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Why this change is needed

The previous version mixed Markdown conventions with RST, which caused:

• Broken or mis-rendered code examples in Sphinx
• Confusing Python snippets that would not run as written
• A Context Settings test example that referenced output not produced by the command

These issues make the guide harder to trust and reuse, especially for new users learning Click testing patterns.

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What this PR does not do

• ❌ Does not change Click behavior
• ❌ Does not introduce new testing patterns
• ❌ Does not refactor existing examples beyond formatting and correctness

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Verification

• Examples now render correctly in Sphinx
• Python snippets are syntactically valid and consistent
• Context Settings behavior is accurately described

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If helpful, I’m happy to follow up with similar cleanups in other testing or docs sections.

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[davidism]

Stop using LLMs to write PRs. Especially stop using them and not even attempting to think for yourself about the result.