Core: Make PlanTableScanResponse buider withCredentials replace instead of append

[dramaticlly]

Follow up of #17638 (comment) with changes in PlanTableScanResponse.Builder and FetchPlanningResultResponse.Builder

1. withCredentials(List<Credential>) now replaces the builder's credentials instead of appending to them.
2. It now rejects a null list and null elements as the follow up of Core: Add storage credentials to FetchPlanningResultResponse #14994 (comment)
3. build() now passes ImmutableList.copyOf(credentials) instead of the builder's live ArrayList.

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AI Disclosure

• Model: Claude Opus 5
• Platform/Tool: Claude Code
• Human Oversight: reviewed
• Prompt Summary: Investigate the withCredentials append-vs-replace question raised in review of Core: Reduce visibility on scan planning response builder for specsById and deleteFiles #17638.

[dramaticlly]

@singhpk234 @nastra @huaxingao if you want to take a look

[nastra]

do we actually need this check? We should get this for free when doing this.credentials = ImmutableList.copyOf(credentialsToAdd)

[dramaticlly]

I think ImmutableList.copyOf(credentialsToAdd) will throw NPE instead of IllegalArgumentException instead, from AGENTS.md seem to favor "Preconditions.checkArgument over NPE"

and I saw we have some precedence in

iceberg/core/src/main/java/org/apache/iceberg/rest/responses/ListTablesResponse.java

Line 84 in 7f879b1

Preconditions.checkArgument(!toAdd.contains(null), "Invalid table identifier: null");

and

iceberg/core/src/main/java/org/apache/iceberg/rest/responses/ListNamespacesResponse.java

Line 83 in 7f879b1

Preconditions.checkArgument(!toAdd.contains(null), "Invalid namespace: null");

[singhpk234]

per

Suggested change

	Preconditions.checkArgument(null != newCredentials, "Invalid credentials: null");
	Preconditions.checkArgument(newCredentials, "Invalid credentials list : null");

iceberg/core/src/main/java/org/apache/iceberg/rest/responses/ListTablesResponse.java

Line 83 in 7f879b1

Preconditions.checkNotNull(toAdd, "Invalid table identifier list: null");

[dramaticlly]

Thanks @singhpk234 , I think ListTableResponse is relative old (merged Feb 2022) compare to the latest agents.md guideline in

iceberg/AGENTS.md

Line 121 in f80a72f

- Use `ConcurrentMap` for shared mutable state. `Preconditions.checkArgument` over NPE.

which prefer IllegalArgumentException over NPE. Let me know if you want to switch to NPE instead