Fix --use-pr-number for changelog add

[lcawl]

Impetus

The --use-pr-number option is no longer working. The filename output from "docs-builder changelog add" is unchanged when that option is present.

Problem

Before this PR, the following command (with the config file from elastic/elasticsearch#140141):

./docs-builder changelog add --prs 140034 \
  --repo elasticsearch --owner elastic \
  --products "elasticsearch 9.3.0" \
  --output /GitHub/elasticsearch/docs/changelog/ \
  --type bug-fix --config /GitHub/elasticsearch/docs/changelog.yml --use-pr-number

... incorrectly generated a file called 1768517811-esql-fix-extent-reading-when-missing.yaml.
This is a regression from the behaviour implemented in #2422.

With this PR fix, it once again generates a file called 140034.yaml.

Solution

The --use-pr-number option wasn't working because:

1. Filename generation ignored UsePrNumber: In CreateSingleChangelog (lines 388-393), the code always generated timestamp-based filenames ({timestamp}-{slug}.yaml) without checking input.UsePrNumber.

2. Missing property copy for multiple PRs: In CreateChangelogsForMultiplePrs (lines 131-149), when creating a copy of the input for each PR, the UsePrNumber property wasn't being copied, so it was lost when processing multiple PRs.

Fixes Applied

1. Updated filename generation (lines 381-410): Added logic to check input.UsePrNumber. When true and a PR URL is provided, it extracts the PR number using the existing ExtractPrNumber method and uses {prNumber}.yaml as the filename. Falls back to the timestamp-based format if extraction fails.

2. Fixed property copy (line 148): Added UsePrNumber = input.UsePrNumber to the prInput copy in CreateChangelogsForMultiplePrs so the flag is preserved when processing multiple PRs.

The build completed successfully with no errors. Your command should now create a file named 140034.yaml instead of the timestamp-based filename.

Testing

• Added a test CreateChangelog_WithUsePrNumber_CreatesFileWithPrNumberAsFilename in tests/Elastic.Documentation.Services.Tests/ChangelogServiceTests.cs to verify that when --use-pr-number is used, the changelog filename is the PR number (e.g., 140034.yaml) instead of a timestamp-based name.

The test:

1. Sets up a mock GitHub PR service that returns PR info for PR #140034
2. Creates a ChangelogInput with UsePrNumber = true and a PR URL
3. Calls CreateChangelog to generate the changelog file
4. Asserts that exactly one file is created
5. Verifies the filename is 140034.yaml (not a timestamp-based name like 1768517811-esql-fix-extent-reading-when-missing.yaml)
6. Verifies the YAML content is correct

The test:

• Compiles successfully
• Passes when executed
• Has no linter errors
• Follows the existing test patterns in the file

This ensures the --use-pr-number option works as expected and prevents regressions.

Generative AI disclosure

1. Did you use a generative AI (GenAI) tool to assist in creating this contribution?

• Yes
• No

4. If you answered "Yes" to the previous question, please specify the tool(s) and model(s) used (e.g., Google Gemini, OpenAI ChatGPT-4, etc.).

Tool(s) and model(s) used: composer-1