Separate built-in parsers from msgspec optional dependency; fix MsgspecParser type handling and conflict detection

[Copilot]

confkit.ext.parsers unconditionally imported msgspec at module load time, breaking INI-only usage without the optional extra. MsgspecParser._parse_value also corrupted string values like "001" → 1 (int), and scalar-vs-section path conflicts were silently dropped on writes.

Parser module split

• New confkit/parsers.py holds ConfkitParser (Protocol), IniParser, EnvParser — zero optional deps
• confkit/ext/parsers.py now contains only MsgspecParser; still raises ImportError immediately without msgspec
• config.py and all tests/examples updated to import built-in parsers from confkit.parsers

from confkit.parsers import IniParser, EnvParser, ConfkitParser  # no msgspec needed
from confkit.ext.parsers import MsgspecParser  # requires msgspec extra

MsgspecParser type handling (replaces _parse_value)

Config._set now passes BaseDataType objects directly to parsers instead of converting to str first. MsgspecParser.set stores native Python types when str(native) == str(data_type) (standard types), and the custom string representation when they differ (e.g. Hex(10) → "0xa", not 10). This preserves round-trip correctness without guessing types from strings.

IniParser.set handles % escaping internally (moved from Config._sanitize_str).

Scalar conflict detection

_navigate_to_section(create=True) now raises ConfigPathConflictError when the final path element is already a scalar — previously only intermediate elements triggered this, silently dropping writes.

Other fixes

• test_msgspecparser_no_msgspec.py: uses match=re.escape(expected_msg) in pytest.raises
• .github/copilot-instructions.md: corrected Parser facade reference → ConfkitParser protocol in parsers.py
• examples/nested_config.py: removed deprecated set_parser(IniParser()) call (auto-detected from .ini extension)

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[Copilot]

Pull request overview

This PR restructures confkit’s parser modules to avoid importing the optional msgspec dependency for INI/.env-only usage, and refines MsgspecParser write behavior (type preservation + stricter path conflict detection).

Changes:

• Moved built-in parsers (ConfkitParser, IniParser, EnvParser) into a new zero-optional-deps module: confkit.parsers.
• Updated Config + tests/examples to import built-in parsers from confkit.parsers and only import MsgspecParser from confkit.ext.parsers when needed.
• Updated MsgspecParser.set() to store native Python types when safe, and raise on scalar-vs-section conflicts even at the final path element.

Reviewed changes

Copilot reviewed 16 out of 16 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
tests/test_two_instances.py	Update import path for IniParser to confkit.parsers.
tests/test_pydantic_models.py	Update import path for IniParser to confkit.parsers.
tests/test_nested_config.py	Split imports: MsgspecParser from confkit.ext.parsers, IniParser from confkit.parsers.
tests/test_multiple_configurations.py	Update import path for IniParser to confkit.parsers.
tests/test_msgspecparser_no_msgspec.py	Tighten ImportError assertion using re.escape() for exact message matching.
tests/test_metaclass.py	Update import path for IniParser to confkit.parsers.
tests/test_env_parser.py	Update import path for EnvParser to confkit.parsers.
tests/test_config_detect_parser.py	Split imports: MsgspecParser from confkit.ext.parsers, IniParser from confkit.parsers.
tests/test_config_decorators.py	Update import path for IniParser to confkit.parsers.
tests/test_config_classvars.py	Update import path for IniParser to confkit.parsers.
tests/test_config.py	Update import path for IniParser to confkit.parsers.
src/confkit/parsers.py	New module for built-in parsers/protocol without optional dependencies; adds % escaping to IniParser.set().
src/confkit/ext/parsers.py	Now msgspec-only; improves _navigate_to_section conflict detection and stores native vs stringified BaseDataType values in set().
src/confkit/config.py	Switch built-in parser imports to confkit.parsers; change _set() to pass values directly to parsers (no longer pre-sanitizing to str).
examples/nested_config.py	Remove deprecated set_parser(IniParser()) usage; rely on auto-detection from .ini.
.github/copilot-instructions.md	Docs update to reflect ConfkitParser protocol and built-in parser module locations.