feat: datetime expansion

[jjcfrancisco]

Description

Adds support for abbreviated datetime formats (year, year-month, date) via a new Search::normalize_datetimes() method. This allows users to opt-in to expanding partial dates to full RFC 3339 datetime ranges - follows similar functionality to pystac.

Key changes:

• Implemented expand_datetime_to_start() and expand_datetime_to_end() helper functions to convert abbreviated dates to full datetime ranges
• Added Search::normalize_datetimes() method for opt-in datetime normalization
• Added two new error variants: InvalidYear and UnrecognizedDateFormat for better error handling
• Comprehensive documentation and test coverage

Sidecar changes:
Bumped assert_cmd from 2.0 to 2.1 in workspace dependencies. The locked version (2.0.14) was missing the cargo_bin_cmd!() macro (see docs), causing compilation failures in test code that relied on it

Related issues

• Closes #463

Checklist

• Unit tests
• Documentation, including doctests
• Pull request title follows conventional commits
• Pre-commit hooks pass (prek run --all-files)

[gadomski]

Thanks for this! This will be a really helpful addition.

As demonstrated by the tests, I don't think we should add this functionality to Items::valid, since that's used in our servers as well, which need to be strict about datetime format. I think we want this only for search, so probably on stac::api::Search. Maybe something like:

impl Search {
    pub fn normalize_datetimes(mut self) -> Result<Search> {
        todo!();
    }
}

[jjcfrancisco]

Thanks for this! This will be a really helpful addition.

As demonstrated by the tests, I don't think we should add this functionality to Items::valid, since that's used in our servers as well, which need to be strict about datetime format. I think we want this only for search, so probably on stac::api::Search. Maybe something like:

impl Search {
    pub fn normalize_datetimes(mut self) -> Result<Search> {
        todo!();
    }
}

I realised my approach had an issue after the CI ran, hopefully this fixes it. I've moved the datetime expansion to Search::normalize_datetimes() as you suggested and reverted items.rs to original. Let me know if you'd like any adjustments!