Simplify defaultConnection fetch and backward-compat env vars

[kmcginnes]

Parent PRD

• Parent Simplify client-server relationship by removing proxy URL from connection config #1618

What to build

Update fetchDefaultConnection to use ${apiBase}defaultConnection as the single fetch URL, removing the SageMaker fallback URL guessing. Update the DefaultConnectionData schema to accept both old field names (GRAPH_EXP_PUBLIC_OR_PROXY_ENDPOINT, GRAPH_EXP_USING_PROXY_SERVER) and the canonical GRAPH_EXP_CONNECTION_URL. Update mapToConnection to handle both formats. Update process-environment.sh to support old Docker env var names (PUBLIC_OR_PROXY_ENDPOINT, USING_PROXY_SERVER) with deprecation warnings, mapping them to the new canonical names.

Type: HITL — requires review before completion.

Acceptance criteria

• fetchDefaultConnection fetches from ${apiBase}defaultConnection only (no SageMaker fallback)
• DefaultConnectionData schema accepts old field names alongside new ones
• mapToConnection produces correct ConnectionConfig from both old and new defaultConnection.json formats
• process-environment.sh maps old env var names to new canonical names
• process-environment.sh logs deprecation warnings when old env var names are used
• GRAPH_EXP_CONNECTION_URL is the canonical field name for the database URL
• Tests verify schema parsing for both old and new field names
• Tests verify mapToConnection output for both formats

Blocked by

• Blocked by Create apiBase module and wire client fetch to /api/* #1621
• Blocked by Simplify ConnectionConfig — remove url and proxyConnection #1622

User stories addressed

• User story 3
• User story 4
• User story 5