fix typos in XML doc comments, ADR docs, and test comments

[rinceyuan]

Motivation & Context

Fix minor typos and leftover template text in documentation and XML doc comments.

Description & Review Guide

• Fix double period (..) in AnthropicClientExtensions.cs XML param docs (lines 23, 77)
• Fix double period (..) in IScopedContentProcessor.cs XML param doc (line 20)
• Fix similar the the → similar to the in ADR 0009 (line 1092)
• Fix reponse → response in ADR 0001 (line 142)
• Fix retreive → retrieve in ChatClientAgentTests.cs (line 467)
• Remove leftover template placeholder text from ADR 0001 and 0006 frontmatter date fields

Related Issue

N/A (documentation hygiene)

Contribution Checklist

• The code builds clean without any errors or warnings
• All unit tests pass, and I have added new tests where possible
• The PR follows the Contribution Guidelines
• This is not a breaking change.

[Copilot]

Pull request overview

This PR performs documentation hygiene across .NET code comments and ADR markdown by correcting typos/punctuation and removing leftover template placeholder text in ADR frontmatter dates.

Changes:

• Fix spelling/punctuation issues in XML doc comments and test comments (e.g., double periods and “retreive” → “retrieve”).
• Correct typos in ADR content (“reponse” → “response”, “similar the the” → “similar to the”).
• Remove template placeholder text from ADR frontmatter date fields while keeping the intended dates.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
dotnet/tests/Microsoft.Agents.AI.UnitTests/ChatClient/ChatClientAgentTests.cs	Fixes a typo in a test XML summary comment (“retrieve”).
dotnet/src/Microsoft.Agents.AI.Purview/IScopedContentProcessor.cs	Removes an extra period in an XML <param> doc comment.
dotnet/src/Microsoft.Agents.AI.Anthropic/AnthropicClientExtensions.cs	Removes extra periods in XML <param> doc comments for client.
docs/decisions/0009-support-long-running-operations.md	Fixes a grammatical typo in ADR prose (“similar to the”).
docs/decisions/0006-userapproval.md	Removes template placeholder text from ADR frontmatter date.
docs/decisions/0001-agent-run-response.md	Removes template placeholder text from frontmatter date and fixes “response” typo in ADR prose.

[rinceyuan]

@microsoft-github-policy-service agree company=Microsoft

[rinceyuan]

@moonbox3 @giles17 This is a small documentation-hygiene PR (typo/punctuation fixes only, no functional changes). Would appreciate a review when you get a chance. Thanks!