Implements the API proposed in #3291.

What's new

Adds four methods to StringAssertions<TAssertions>, matching the proposal in the issue:

public AndConstraint<TAssertions> HaveLineCount(int expected, string because = "", params object[] becauseArgs);
public AndConstraint<TAssertions> NotHaveLineCount(int unexpected, string because = "", params object[] becauseArgs);
public AndConstraint<TAssertions> ContainLine(string expectedLine, string because = "", params object[] becauseArgs);
public AndConstraint<TAssertions> NotContainLine(string unexpectedLine, string because = "", params object[] becauseArgs);

string log = "Starting up\nConnected to database\nReady";

log.Should().HaveLineCount(3);
log.Should().ContainLine("Connected to database");
log.Should().NotContainLine("Error");

• Lines are split using \r\n, \n, and \r as line separators (all three treated as equivalent), consistent with line-ending handling already used internally elsewhere in the library.
• ContainLine/NotContainLine match whole lines only (not substrings), and guard against a null expected/unexpected line via Guard.ThrowIfArgumentIsNull (empty string is a valid line to search for).
• HaveLineCount/NotHaveLineCount fail explicitly with a <null>-aware message when the subject string is null, following the same pattern as HaveLength.

Tests

Added spec files under Tests/FluentAssertions.Specs/Primitives/:

• StringAssertionSpecs.HaveLineCount.cs
• StringAssertionSpecs.NotHaveLineCount.cs
• StringAssertionSpecs.ContainLine.cs
• StringAssertionSpecs.NotContainLine.cs

Covering success/failure paths, null subjects, mixed line endings, and argument validation.

Housekeeping

• Updated Tests/Approval.Tests/ApprovedApi/FluentAssertions/*.verified.txt via AcceptApiChanges.ps1 for the new public API surface.
• Updated docs/_pages/releases.md and docs/_pages/strings.md.
• Full FluentAssertions.Specs suite (6003 tests) and approval tests pass locally.

Closes #3291 (pending maintainer API approval).