feat(data-access): rework Site contentAiConfig (name + index)

[duynguyen]

What

Reworks the Site contentAiConfig in spacecat-shared-data-access:

• Adds an optional name (non-empty when present) to the contentAiConfig schema.
• Keeps the legacy index field as an optional, first-class field so pre-existing configs validate without warnings.
• Adds updateContentAiConfig({ name, index }) — name and index are equal citizens: both are independently writable and optional, each overwritten only when provided, so updating one never clobbers the other (and existing values are preserved via merge).
• getContentAiConfig() and Config.toDynamoItem() serialization are unchanged (pass-through).

Contract

contentAiConfig, name, and index are all optional:

Input	Result
omitted	valid (getContentAiConfig() is undefined)
{}	valid
{ name }	valid
{ index } (legacy)	valid
{ name, index }	valid, both round-trip
{ name: ' ' }	rejected (present but blank)
unknown field	rejected

updateContentAiConfig(...):

Call	Effect
{ name }	sets/overwrites name, preserves existing index
{ index }	sets/overwrites index, preserves existing name
{ name, index }	sets/overwrites both
() (no args)	leaves config unchanged

Notes

• name is intentionally optional (not required) so existing persisted configs — including ones that only carry index — validate cleanly and emit no warnings. Downstream readers of getContentAiConfig().name must tolerate undefined.
• No data backfill is included; legacy index-only records are preserved as-is.

Testing

• Unit tests updated in test/unit/models/site/config.test.js covering the optional-field matrix, blank-name rejection, updateContentAiConfig name-only / index-only / both / no-arg cases, merge preservation, and toDynamoItem round-trip.
• Full package suite passes; coverage thresholds met.

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