fix: AWS endpoint scheme/region + default-region selector

[fuleinist]

Summary

Connecting to a real AWS DynamoDB endpoint via the generic GUI connection form (host / port / username / password) was broken in normalized_params — only DynamoDB Local worked. This PR fixes the transport/signing bugs and adds a region selector.

1. Wrong scheme — http://host:443

host + port always became http://host:port. With host=dynamodb.us-west-2.amazonaws.com, port=443 the plugin dialed plain HTTP to a TLS-only port → transport-level dispatch failure.

Fix: HTTPS when port == 443 or host ends with .amazonaws.com.

2. Wrong signing region — hardcoded us-east-1

The generic form has no region field, so the signing region defaulted to us-east-1 regardless of the endpoint. SigV4 signed for the wrong region → AWS rejects every request with InvalidSignatureException.

Fix: parse the region from the endpoint hostname (dynamodb.us-west-2.amazonaws.com → us-west-2).

3. Region selector (new)

A per-connection region dropdown in the connection edit page is not possible from a driver plugin today: the connection-modal.connection_content slot only receives {driver, database, onDatabaseChange, connectionName} and only renders for no_connection_required drivers. (App-side follow-up: extend the slot context with form state, or add a driver-declared connection-fields mechanism.)

Instead this PR declares a plugin-level "Default AWS region" select in .tabularium (all 34 current regions per the AWS Regions doc). The host renders it under Settings → Plugins → DynamoDB and delivers the saved value via the initialize RPC, which the plugin now stores (previously ignored).

Region resolution order:

1. explicit region param
2. region parsed from an AWS endpoint hostname
3. plugin-level default-region setting
4. us-east-1

Changes

• normalized_params: scheme selection + region_from_endpoint helper + settings fallback
• new settings module: stores initialize settings (process-global)
• .tabularium: settings array with the region select
• 8 new regression tests (https rules, region parsing, settings fallback, precedence). Settings tests share a lock — the cell is process-global.

Test plan

• cargo test — 185 passed (8 new)
• cargo clippy / cargo fmt — clean
• Live verification against a real AWS account using exactly what the GUI form sends (host/port/username/password only):
  • test_connection → {"success": true} (previously failed)
  • get_tables → 400 tables in ~6.5s with full metadata
  • execute_query → rows + complete pagination object

No credentials, endpoints, or account-specific data included in this change.

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