feat(athena): AthenaDbConfig profile schema

[fpiped]

Resolves #16252
Related: #13822

Problem

type: athena in profiles.yml fails at parse time (InvalidConfig (dbt1005): Failed to parse profiles.yml: unknown variant 'athena'), before the experimental-adapter gate and before dbt-auth are reached. The Athena pieces already in the repo (AdapterType::Athena, Backend::Athena, the ADBC option keys in crates/dbt-adbc/src/athena.rs, AthenaAuth in crates/dbt-auth/src/athena/mod.rs) are therefore unreachable from a profile: DbConfig has no Athena variant (crates/dbt-schemas/src/schemas/profiles.rs, // Athena,).

Solution

The profile-schema slice, same shape as ClickHouse "Part 2" (b468efcd6), plus one arm in the relation factory:

• AthenaDbConfig, with the field set of dbt-athena's AthenaCredentials (connections.py) and the catalog alias for database. Every dbt-athena field is declared, including the ones AthenaAuth currently rejects (assume_role_*, s3_data_dir, poll_interval, ...): the auth layer reads the profile through to_mapping(), so a field absent from the struct would be dropped silently instead of producing the existing "not yet supported" error. A test pins this.
• DbConfig::Athena wired through adapter_type, get_unique_field (s3_staging_dir, as in dbt-athena), get_database / get_database_or_default (defaults to awsdatacatalog, the default AthenaAuth applies), get_schema, get_threads / set_threads, to_yaml_value.
• connection_keys_for(AdapterType::Athena): dbt-athena's _connection_keys restricted to what AthenaAuth honours today; aws_secret_access_key and aws_session_token stay out, so dbt debug never prints them (covered by the existing connection_keys_never_expose_a_credential).
• TargetContext::Athena / AthenaTargetEnv (region_name, s3_staging_dir, work_group, aws_profile_name plus the common database / schema / type / threads). Threads parsing goes through a small threads_from_profile helper instead of a fourth inline copy of the same match.
• relation/factory.rs: Athena joins the generic RelationStatic arm, so dbt parse / dbt ls stop at a configuration error instead of todo!("Athena").
• dbt-test-utils::profiles: schema / database overrides for Athena, as for the other adapters.

Athena is not added to NON_EXPERIMENTAL_ADAPTERS, so it stays behind DBT_ALLOW_EXPERIMENTAL_ADAPTERS. The remaining todo!("Athena") arms in dbt-adapter (adapter_impl.rs metadata adapter, column_builder.rs, sql_types.rs) and the dbt-athena macro package under dbt_macro_assets are the next slices and are not touched here.

Behaviour with this branch, debug build, against the profile below:

Command	Result
dbt debug, DBT_ALLOW_EXPERIMENTAL_ADAPTERS=false	The 'athena' adapter is not yet supported by dbt. Supported adapters: snowflake, ... (gate reached; before: unknown variant 'athena')
dbt debug, gate open	AuthenticationFailed: Driver not found: adbc_driver_athena (profile → AthenaAuth → driver load)
dbt debug, catalog: my_catalog	same as above; alias accepted
dbt debug, assume_role_arn: ...	Athena profile field 'assume_role_arn' is not yet supported by the dbt Athena backend (the AthenaAuth rejection now fires)
dbt debug, no schema	Athena requires 'schema' in profile configuration
dbt parse / dbt ls, gate open	Missing dbt_project.yml in embedded package 'dbt-athena' (before this branch: panic at relation/factory.rs:28)

athena_probe:
  target: dev
  outputs:
    dev:
      type: athena
      region_name: eu-west-1
      s3_staging_dir: s3://example-bucket/athena-results/
      schema: analytics
      work_group: primary

Not in scope, and a dbt Labs step: the Athena ADBC driver is not in INSTALLABLE_DRIVERS, so loading falls back to LoadStrategy::System (a locally built adbc_driver_athena from dbt-labs/athena).

Checks run locally: cargo fmt --check, cargo nextest run -p dbt-schemas -E 'test(/profiles::tests/)' (36 passed), cargo check -p dbt-test-utils -p dbt-profile-schemas -p dbt-init -p dbt-loader -p dbt-compilation, cargo clippy --all-targets --all-features on dbt-schemas, dbt-test-utils and dbt-adapter, cargo build -p dbt-sa-cli for the table above.

Checklist

• I have read the contributing guide and understand what's expected of me.
• I have run this code in development, and it appears to resolve the stated issue.
• This PR includes tests, or tests are not required or relevant for this PR.
• This PR has no interface changes (e.g., macros, CLI, logs, JSON artifacts, config files, adapter interface, etc.) or this PR has already received feedback and approval from Product or DX. — adds the type: athena profile type, gated as experimental; flagging for Product/DX review.

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