fix(init): validate version constraint in interactive prompt

[SarthakB11]

Pull Request Check List

Resolves: #8797

• Added tests for changed code.
• Updated documentation for changed code.

The version-constraint prompt in poetry init (and poetry add -i) only stripped whitespace, so invalid PEP 440 strings like latest, 1.0,!2.0, or ==1., were accepted and written verbatim into pyproject.toml. The error only surfaced at the next poetry lock / install.

The validator now calls poetry-core's parse_constraint and raises ValueError("Invalid version constraint: <value>") on failure, chained from the underlying parser error via from e. The existing set_max_attempts(3) re-prompts the user. Same pattern as _validate_author / _validate_package in the same file. Blank input still returns None (skip).

Hard rejection (vs the reporter's suggestion of "ask for confirmation") was chosen for consistency with the two other validators in init.py. Tests in tests/console/commands/test_init.py cover valid, invalid, and blank inputs.

[sourcery-ai]

Hey - I've found 1 issue

Prompt for AI Agents

Please address the comments from this code review:

## Individual Comments

### Comment 1
<location path="tests/console/commands/test_init.py" line_range="1222-1233" />
<code_context>
+    [
+        ("^1.0", "^1.0"),
+        ("==1.2.3", "==1.2.3"),
+        (">=1,<2", ">=1,<2"),
+        ("", None),
+        (None, None),
</code_context>
<issue_to_address>
**suggestion (testing):** Add a test case for valid constraints with surrounding whitespace

To cover `_validate_version_constraint`'s stripping behavior, please add a case like `("  ^1.0  ", "^1.0")` to this parametrization so we also test whitespace around a valid constraint, not just pure-whitespace mapping to `None`.

```suggestion
@pytest.mark.parametrize(
    ("constraint", "expected"),
    [
        ("^1.0", "^1.0"),
        ("==1.2.3", "==1.2.3"),
        (">=1,<2", ">=1,<2"),
        ("  ^1.0  ", "^1.0"),
        ("", None),
        (None, None),
        ("   ", None),
    ],
)
def test_validate_version_constraint_accepts_valid_inputs(
```
</issue_to_address>

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[sourcery-ai]

suggestion (testing): Add a test case for valid constraints with surrounding whitespace

To cover _validate_version_constraint's stripping behavior, please add a case like (" ^1.0 ", "^1.0") to this parametrization so we also test whitespace around a valid constraint, not just pure-whitespace mapping to None.

Suggested change

	@pytest.mark.parametrize(
	("constraint", "expected"),
	[
	("^1.0", "^1.0"),
	("==1.2.3", "==1.2.3"),
	(">=1,<2", ">=1,<2"),
	("", None),
	(None, None),
	(" ", None),
	],
	)
	def test_validate_version_constraint_accepts_valid_inputs(
	@pytest.mark.parametrize(
	("constraint", "expected"),
	[
	("^1.0", "^1.0"),
	("==1.2.3", "==1.2.3"),
	(">=1,<2", ">=1,<2"),
	(" ^1.0 ", "^1.0"),
	("", None),
	(None, None),
	(" ", None),
	],
	)
	def test_validate_version_constraint_accepts_valid_inputs(