fix: handle @latest version specifier with environment markers

[dajiaohuang]

Resolves #10227

Problem

When using poetry add with @latest version specifier combined with environment markers (e.g., poetry add --lock "poetry@latest ; platform_machine!=\"aarch64\""), the command would fail with the error:

```
Path /path/to/project/latest for poetry does not exist
```

This happened because:

1. The regex stripping @latest in init._parse_requirements() only matched at end-of-string (`@\s*latest$`), failing when markers followed
2. The _parse_pep508 detection regex did not account for the @latest pattern, causing invalid PEP 508 parsing to be attempted, which then fell through to path parsing (treating "latest" as a directory)

Fix

• Updated the @latest stripping regex in both init.py and RequirementsParser.parse() to handle cases where @latest is followed by a semicolon (environment markers), while preserving the marker portion: `@\slatest(\s;|$)`
• Added the @latest detection pattern to _parse_pep508() for robustness
• Added test cases covering @latest with and without extras and markers

Test cases added

• `demo@latest`
• `demo[a,b]@latest`
• `demo@latest ; python_version >= "3.8"`
• `demo[a,b]@latest ; python_version >= "3.8"`

All existing tests continue to pass.

[sourcery-ai]

Hey - I've reviewed your changes and they look great!

Sourcery assessment

Approved.

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[dimbleby]

Are you making @latest just do the same as not typing that in the first place? I don't really see the point, we should instead ask people not to type that in the first place.

Added the @latest detection pattern to _parse_pep508() for robustness

this is definitely wrong, a PEP508 parser should fail to parse non-compliant strings

[kokokoXUY]

I found a reproducible VCS revision regression in the new @latest normalization; details inline.

[kokokoXUY]

This unconditional rewrite also matches a legitimate VCS revision named latest. I reproduced RequirementsParser.parse("git+https://github.com/demo/demo.git@latest") at this PR head (bce8925): it returns {'git': 'https://github.com/demo/demo.git'} with no rev, whereas the unchanged base returns rev: 'latest'. That silently installs the default branch instead of the requested revision. The same rewrite in init.py affects poetry add too. Could the @latest normalization be limited to a package-name version specifier, leaving VCS URLs and direct references untouched? A regression test for a Git URL with @latest would guard this.