Add unit tests for ExtractedTextFormatter

[gaoxiaolei-s59]

ExtractedTextFormatter in spring-ai-commons implements the text clean-up pipeline used by the document readers (PDF, Tika) — blank-line consolidation, left alignment, top/bottom line deletion with page-skip and custom line separators — but has no dedicated unit tests today (it is only exercised indirectly through document-reader tests).

This PR adds ExtractedTextFormatterTests (19 tests, JUnit 5 + AssertJ, no mocks) covering:

• trimAdjacentBlankLines: consecutive blank lines collapsed, space-only lines treated as blank, single blank line preserved
• alignToLeft: leading-space stripping, inner space-run collapsing, trailing-space removal
• deleteBottomTextLines / deleteTopTextLines: requested-line removal, more-lines-than-present edge cases, blank/null passthrough, custom (\r\n) separators
• format(...): default blank-line-only behavior, left alignment, top/bottom deletion, numberOfTopPagesToSkipBeforeDelete threshold behavior across page numbers, no-arg format treating the page as page 0, and the combined pipeline
• builder/defaults() returning fresh instances

Verification:

./mvnw -pl spring-ai-commons test -Dtest=ExtractedTextFormatterTests
Tests run: 19, Failures: 0, Errors: 0, Skipped: 0
BUILD SUCCESS

spring-javaformat:validate passes and the commit is signed off per the DCO.