Introduce CassStr* abstractions

[wprzytula]

Fixes: #301
Based on: #448

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What's done

This PR introduces FFI abstractions for C string types:

• CassStrNulTerminated - represents a nul-terminated C string passed by a pointer;
• CassStrLenDelimited - represents a string slice passed by a pointer, paired with
• CassStrLen - represents length of CassStrLenDelimited. I decided to make a newtype for this to increase type safety.

All abstractions are repr(transparent), and they appear in FFI functions' signatures. They provide convenient constructors and conversion methods.

What we gain

Type safety

It's now harder to misuse one of string pointer as another.

Borrow checks

Both CassStrNulTerminated and CassStrLenDelimited carry a lifetime, which prevents UAFs.

Self-documenting code

The code is much more explicit and clear in what it does with strings.

The names are a bit verbose, but this is in line with our FFI approach (see long names of CassPtr abstractions, such as CassBorrowedSharedPtr).

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Pre-review checklist

• I have split my patch into logically separate commits.
• All commit messages clearly explain what they change and why.
• PR description sums up the changes and reasons why they should be introduced.
• I have implemented Rust unit tests for the features/changes introduced.
• [ ] I have enabled appropriate tests in Makefile in {SCYLLA,CASSANDRA}_(NO_VALGRIND_)TEST_FILTER.
• I added appropriate Fixes: annotations to PR description.

[Copilot]

Pull request overview

This PR introduces CassStr* wrapper types to improve/standardize C-string ↔ Rust &str conversions across the Rust wrapper’s FFI surface, addressing the inability of the previous helpers to distinguish null pointers from invalid UTF-8 (Issue #301). It updates exported APIs and tests to use these abstractions and starts mapping conversion failures to more explicit error paths.

Changes:

• Add CassStrWithNul, CassStrLenDelimited, CassStrLen, and PtrToStrError abstractions and migrate many FFI functions from raw *const c_char/size_t.
• Replace ptr_to_cstr(_n) usage with typed conversions, improving error differentiation and propagating failures more intentionally in several call sites.
• Update integration/unit tests and test utilities to use the new string wrapper APIs.

Reviewed changes

Copilot reviewed 23 out of 23 changed files in this pull request and generated 16 comments.

Show a summary per file

File	Description
scylla-rust-wrapper/src/argconv.rs	Introduces CassStr* types and PtrToStrError; updates array-to-string helper.
scylla-rust-wrapper/src/binding.rs	Migrates bind-by-name APIs to CassStr* and tightens null/empty/non-UTF8 handling.
scylla-rust-wrapper/src/cass_error.rs	Changes cass_error_desc to return CassStrWithNul.
scylla-rust-wrapper/src/cluster.rs	Migrates many cluster APIs to CassStr* and updates parsing/validation logic.
scylla-rust-wrapper/src/cql_types/mod.rs	Changes string-returning helpers to return CassStrWithNul.
scylla-rust-wrapper/src/cql_types/data_type.rs	Migrates type-name/keyspace/class-name setters and related name-based APIs to CassStr*.
scylla-rust-wrapper/src/cql_types/inet.rs	Migrates inet-from-string APIs to CassStr*.
scylla-rust-wrapper/src/cql_types/user_type.rs	Removes now-unneeded raw c_char import (cleanup during migration).
scylla-rust-wrapper/src/cql_types/uuid.rs	Migrates uuid-from-string APIs to CassStr*.
scylla-rust-wrapper/src/exec_profile.rs	Migrates exec-profile name parameters to CassStr* with improved error handling.
scylla-rust-wrapper/src/logging.rs	Updates log level/file string handling to use CassStrWithNul.
scylla-rust-wrapper/src/metadata.rs	Migrates metadata “by name” lookups to CassStr*.
scylla-rust-wrapper/src/query_result.rs	Migrates column-by-name lookups to CassStr* and adds null/empty handling tests.
scylla-rust-wrapper/src/runtime.rs	Updates runtime tests to use CassStrLenDelimited/CassStrLen.
scylla-rust-wrapper/src/session.rs	Migrates connect/prepare APIs to CassStr* and updates unit tests accordingly.
scylla-rust-wrapper/src/ssl.rs	Migrates SSL PEM input APIs to CassStr* and adds UTF-8 validation path.
scylla-rust-wrapper/src/statements/prepared.rs	Migrates prepared-parameter-by-name APIs to CassStr* with explicit error logging.
scylla-rust-wrapper/src/statements/statement.rs	Migrates statement creation/host setting to CassStr* and adds null-pointer binding tests.
scylla-rust-wrapper/src/testing/integration.rs	Updates integration stubs to use CassStr* types.
scylla-rust-wrapper/src/testing/utils.rs	Updates test helpers/macros to use CassStr* conversions.
scylla-rust-wrapper/tests/integration/consistency.rs	Updates integration tests to call APIs with CassStrWithNul/CassStrLen*.
scylla-rust-wrapper/tests/integration/session.rs	Updates integration session tests to use CassStrWithNul and new signatures.
scylla-rust-wrapper/tests/integration/utils.rs	Removes old pointer helpers and updates error/message formatting to new conversions.

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[Copilot]

Pull request overview

Copilot reviewed 23 out of 23 changed files in this pull request and generated 4 comments.

[Copilot]

Pull request overview

Copilot reviewed 23 out of 23 changed files in this pull request and generated no new comments.

Comments suppressed due to low confidence (1)

scylla-rust-wrapper/src/ssl.rs:354

• cass_ssl_set_private_key_n returns CASS_ERROR_SSL_INVALID_CERT when BIO_new_mem_buf fails, but this is a private-key operation and should return CASS_ERROR_SSL_INVALID_PRIVATE_KEY (consistent with the other error paths in this function).

    if bio.is_null() {
        return CassError::CASS_ERROR_SSL_INVALID_CERT;
    }

[wprzytula]

Rebased on master.

[wprzytula]

Made the new CassStr* abstractions newtypes over CassBorrowedSharedPtr.

[wprzytula]

Deleted the commit that introduced the unsafe blocks in the macros.