macros: Add support for C-style enum serialization via #[scylla(repr = "...")]

[ziara1]

Motivation

Users often map C-style enums (enums with unit variants and integer discriminants) to integer columns in ScyllaDB (e.g., tinyint, smallint, int). Currently, users must manually implement SerializeValue and DeserializeValue for these enums, which involves tedious boilerplate for matching variants to integers and vice versa.

Solution

This commit extends SerializeValue and DeserializeValue macros to support C-style enums directly.
Users can now annotate their enum with #[scylla(repr = "TYPE")] (e.g., "i8", "i32"), and the macro will generate code that:

• Serializes the enum variant as its integer discriminant.
• Deserializes an integer from the DB and matches it back to the corresponding enum variant.

Behavior details:

• If a discriminant is not specified explicitly, standard Rust rules apply (starts at 0, increments by 1).
• Deserialization fails with a proper error if the integer value from the DB does not match any known variant.
• Type checking delegates to the underlying integer type (e.g., i32 checks against Int column).

What's done

Refactored DeserializeValue derive

• Refactored deserialize_value_derive to handle Data::Enum separately.
• Added parsing for the #[scylla(repr = "...")] attribute via EnumAttrs.
• Implemented deserialize_value_derive_enum, which generates a match statement mapping integers from the database back to enum variants.

Refactored SerializeValue derive

• Refactored serialize_value_derive to handle Data::Enum.
• Implemented derive_serialize_value_enum, which casts the enum variant to the representation type (e.g., *self as i32) before serialization.

What has been tested

Tests

Added comprehensive tests covering:

• Integer sizes: Support for i8 (TinyInt), i16 (SmallInt), i32 (Int), and i64 (BigInt).
• Discriminants: Tested standard auto-incrementing discriminants and manual discriminants with gaps.
• Error Handling: Verified that deserialization fails with a clear error when the database value doesn't match any enum variant.
• Type Checking: Verified that using an enum wrapper incorrectly (e.g., against a Text column) produces a type check error.

Fixes: #923

Pre-review checklist

• I have split my patch into logically separate commits.
• All commit messages clearly explain what they change and why.
• I added relevant tests for new features and bug fixes.
• All commits compile, pass static checks and pass test.
• PR description sums up the changes and reasons why they should be introduced.
• I have provided docstrings for the public items that I want to introduce.
• I have adjusted the documentation in ./docs/source/.
• I added appropriate Fixes: annotations to PR description.

[github-actions]

cargo semver-checks found no API-breaking changes in this PR.
Checked commit: d79a4c1

[Copilot]

Pull request overview

This PR adds support for deriving SerializeValue and DeserializeValue traits for enums, enabling automatic serialization and deserialization of C-style enums to/from integer CQL types. The implementation converts enum variants to their discriminant values, with configurable representation types.

Key Changes:

• Added enum-specific derive implementations for SerializeValue and DeserializeValue macros
• Introduced repr attribute to specify the underlying integer representation type (defaults to i32)
• Made helper functions pub(crate) to support enum deserialization lifetime handling

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 9 comments.

File	Description
scylla-macros/src/serialize/value.rs	Adds EnumAttrs struct and derive_serialize_value_enum function to generate serialization code for enums, converting them to their integer discriminant values
scylla-macros/src/deserialize/value.rs	Adds EnumAttrs struct and deserialize_value_derive_enum function to generate deserialization code for enums, parsing integer values back to enum variants
scylla-macros/src/deserialize/mod.rs	Exports lifetime helper functions as pub(crate) to enable reuse in enum deserialization

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[Copilot]

Pull request overview

Copilot reviewed 7 out of 7 changed files in this pull request and generated 13 comments.

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[wprzytula]

@ziara1 Please first address clippy's complaint and Copilot's comments, then ping me.

[ziara1]

@wprzytula

[ziara1]

@wprzytula

[wprzytula]

@ziara1 Please squash fixes into original commits.

[ziara1]

@wprzytula done