Move pattern specification for `BaseType.identifier` to the annotation for the `str` type

[candleindark]

My guess of the intention for BaseType.identifier is an object either validates to an AnyHttpUrl or a str matching the pattern of r"^[a-zA-Z0-9-]+:[a-zA-Z0-9-/\._]+$". If my guess is correct, the type annotation should be corrected to the proposed change in this PR.

The current type annotation of BaseType.identifier has problems exhibited by the Foo.identifier from the following example (Baz.identifier exhibits the solution proposed by this PR).

from typing import Optional, Union
import json

from typing_extensions import Annotated

from pydantic import BaseModel, Field, AnyHttpUrl, StringConstraints, ValidationError

PATTERN = r"^[a-zA-Z0-9-]+:[a-zA-Z0-9-/\._]+$"

class Foo(BaseModel):
    identifier: Optional[Union[AnyHttpUrl, str]] = Field(None, pattern=PATTERN)

class Bar(BaseModel):
    identifier: Optional[
        Union[AnyHttpUrl, Annotated[str, StringConstraints(pattern=PATTERN)]]
    ] = None

class Baz(BaseModel):
    identifier: Optional[
        Annotated[
            Union[AnyHttpUrl, Annotated[str, StringConstraints(pattern=PATTERN)]],
            Field(union_mode="left_to_right"),
        ]
    ] = None

# Try inputting an AnyHttpUrl object
print("======= Interation 0 =======")
try:
    foo0 = Foo(identifier=AnyHttpUrl("https://example.com"))
except ValidationError as e:
    print(e)
    """
    1 validation error for Foo
    identifier
      Input should be a valid string [type=string_type, input_value=Url('https://example.com/'), input_type=Url]
        For further information visit https://errors.pydantic.dev/2.7/v/string_type
    """
else:
    raise RuntimeError("Expected a validation error")

bar0 = Bar(identifier=AnyHttpUrl("https://example.com"))
baz0 = Baz(identifier=AnyHttpUrl("https://example.com"))

# Try inputting a string object
print("\n======= Interation 1 =======")
try:
    foo1 = Foo(identifier="https://www.python.org/~guido?arg=1#frag")
except ValidationError as e:
    print(e)
    """
    1 validation error for Foo
    identifier
      String should match pattern '^[a-zA-Z0-9-]+:[a-zA-Z0-9-/\._]+$' [type=string_pattern_mismatch, input_value='https://www.python.org/~guido?arg=1#frag', input_type=str]
        For further information visit https://errors.pydantic.dev/2.7/v/string_pattern_mismatch
    """
else:
    raise RuntimeError("Expected a validation error")

bar1 = Bar(identifier="https://www.python.org/~guido?arg=1#frag")
print(f"type(bar1.identifier): {type(bar1.identifier)}")
"""type(bar1.identifier): <class 'pydantic_core._pydantic_core.Url'>"""

baz1 = Baz(identifier="https://www.python.org/~guido?arg=1#frag")
print(f"type(baz1.identifier): {type(baz1.identifier)}")
"""type(baz1.identifier): <class 'pydantic_core._pydantic_core.Url'>"""

# Try inputting a string object that are simple
print("\n======= Interation 2 =======")
bar2 = Bar(identifier="https://example.com")
print(f"type(bar2.identifier): {type(bar2.identifier)}")
"""type(bar2.identifier): <class 'str'>"""

baz2 = Baz(identifier="https://example.com")
print(f"type(baz2.identifier): {type(baz2.identifier)}")
"""type(baz2.identifier): <class 'pydantic_core._pydantic_core.Url'>"""

Essentially, the current annotation fails to validate any AnyHttpUrl object and any HTTP URL, as a str, that contains a special character.

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