feat(config): Config::validate rules — points_per_unit>0, plan→provider exists, endpoints≥1, protocol enum (+4 tests)

[argszero]

Summary

Closes #6 — config validation rules for the TOML config module (src/config.rs, landed in PR #71's P0-A).

Adds Config::validate() invoked by Config::load(), implementing the validation rules the issue requested:

1. [points] points_per_unit > 0
2. every plan.provider must reference an existing provider (no dangling refs)
3. every plan has at least 1 endpoint
4. endpoint.protocol must be one of the defined enum: openai_chat | anthropic | responses

Tests (4 new, 14 total)

• validate_rejects_zero_points_per_unit
• validate_rejects_missing_provider_ref
• validate_rejects_empty_endpoints
• validate_rejects_illegal_protocol
• parse_config_example_ok now also asserts config.example.toml passes validation

Verification

• cargo test — 14/14 pass
• cargo clippy --all-targets -- -D warnings clean
• cargo fmt --check clean
• README status updated

Related Issue

Closes #6 (feat(config): TOML 配置加载模块 — the parse/load part was delivered in PR #71; this PR adds the 校验 rules + failure-branch tests, completing the issue's scope).