fix(graph): keep streamed tool-call argument fragments that are substrings of accumulated args

[logicwu0]

Closes #4066

Problem

When a model streams a tool call, the argument JSON arrives split across chunks and NodeExecutor.mergeToolArguments() reassembles it. It short-circuited on String.contains():

if (currentArguments.startsWith(previousArguments) || currentArguments.contains(previousArguments)) return currentArguments;
if (previousArguments.startsWith(currentArguments) || previousArguments.contains(currentArguments)) return previousArguments;

An incremental delta can legitimately be a substring of the already-accumulated arguments, and the contains() branch then drops it. Concrete case: streaming {"count": 100} digit-by-digit as {"count": / 1 / 0 / 0 / }. When merging the accumulated {"count": 10 with the final 0, previousArguments.contains("0") is true, so the fragment is dropped and the arguments become {"count": 10}.

This is the same class of failure as #4066 (toolInput cannot be null or empty / lost tool arguments): the merge already handles the name/args split (commit 5c2a0fb), but the contains() heuristic silently corrupts arguments containing repeated characters.

Fix

Only a prefix (startsWith) indicates a cumulative-snapshot chunk (some providers resend the full arguments each chunk) that should replace the accumulator. A mere substring does not. Removing the two contains() checks makes incremental fragments always append, while cumulative-snapshot providers are still handled by startsWith.

Tests

Extends StreamFunctionCallTest (which already covers the split-chunk merge for #4406):

• testStreamingRepeatedDigitFragmentsShouldNotBeDropped — streams {"count": 100} digit-by-digit; asserts {"count": 100} (fails on pre-fix code: produced {"count": 10}).
• testStreamingCumulativeSnapshotFragmentsShouldNotBeConcatenated — a provider resending the full arguments each chunk still collapses to a single {"city":"Beijing"} (guards the retained startsWith path).

All spring-ai-alibaba-graph-core executor tests pass; spotless:apply run.

Note on #4066

The original name/arguments split reported in #4066 is already handled by the current mergeToolCalls implementation; this PR fixes the remaining argument-corruption edge case and adds regression coverage, so #4066 can be closed.