AddFiles: create the table from the file schemas when it does not exist

[claudevdm]

Before this change the pre-pass required an existing table. AddFiles without evolution creates a missing table from whichever file happens to reach ConvertToDataFile first; with the pre-pass every schema of the window is known before anything is created.

Create path (CommitSchemaUnion.create), taken when loadTable throws NoSuchTableException:

• The window's schemas are folded into one union on a scratch create transaction that is never committed (the same staging loop as the evolve path, so a schema conflicting with another is reported or skipped per the handling, and the create is abandoned under FAIL_PIPELINE before anything exists).
• The real table is built directly from createdSchema(union, config): every column optional at every level - one lucky file's declared or proven required-ness must not become a table constraint that the next file violates - except pinned paths and their ancestors, which are created required. Creation is the schema-authoring moment; evolution never tightens columns afterwards. A pin that no file schema carries fails the create under FAIL_PIPELINE (later windows could only add the column optional, so the pin would stay inert forever) and warns under ROUTE_TO_ERRORS. The created table is born with a single schema version.
• Creation is not gated on any particular evolution option: the options guard an existing table's schema, and there is none to guard yet.
• Partition spec, sort order and table properties come from TableCreation (the AddFiles constructor arguments), resolved against the union, so a partition or sort column carried by any schema of the window works; the name mapping is written in the same transaction.
• Create race: two workers (or a concurrent non-evolution AddFiles) may create the table at once. AlreadyExistsException joins CommitFailedException in the retry, and the next attempt takes the evolve path against the table the other party created.
• Empty window against a missing table returns NO_TABLE (-1) and creates nothing.

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[claudevdm]

R: @ahmedabu98

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