[SPARK-54375][CONNECT][TESTS] Add assume to cases in PythonPipelineSuite to skip tests when PyConnect dependencies is not available

### What changes were proposed in this pull request?
SPARK-54020 added some new test cases in `PythonPipelineSuite`. This pr incorporates `assume(PythonTestDepsChecker.isConnectDepsAvailable)` for these test cases to ensure that the tests are skipped rather than failing when PyConnect dependencies are missing.

### Why are the changes needed?
Enhance the robustness of test cases. Prior to this, when executing `build/sbt "connect/testOnly org.apache.spark.sql.connect.pipelines.PythonPipelineSuite"`:

```
[info] - reading internal datasets outside query function that trigger eager analysis or execution will fail (spark.sql("SELECT * FROM src")) *** FAILED *** (4 milliseconds)
[info]   "org.apache.spark.sql.connect.PythonTestDepsChecker.isConnectDepsAvailable was false" did not contain "TABLE_OR_VIEW_NOT_FOUND" (PythonPipelineSuite.scala:546)
[info]   org.scalatest.exceptions.TestFailedException:
[info]   at org.scalatest.Assertions.newAssertionFailedException(Assertions.scala:472)
[info]   at org.scalatest.Assertions.newAssertionFailedException$(Assertions.scala:471)
[info]   at org.scalatest.Assertions$.newAssertionFailedException(Assertions.scala:1231)
[info]   at org.scalatest.Assertions$AssertionsHelper.macroAssert(Assertions.scala:1295)
[info]   at org.apache.spark.sql.connect.pipelines.PythonPipelineSuite.$anonfun$new$43(PythonPipelineSuite.scala:546)
[info]   at org.apache.spark.sql.connect.pipelines.PythonPipelineSuite.$anonfun$new$43$adapted(PythonPipelineSuite.scala:532)
[info]   at org.apache.spark.SparkFunSuite.$anonfun$gridTest$2(SparkFunSuite.scala:241)
[info]   at scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.scala:18)
[info]   at org.scalatest.enablers.Timed$$anon$1.timeoutAfter(Timed.scala:127)
[info]   at org.scalatest.concurrent.TimeLimits$.failAfterImpl(TimeLimits.scala:282)
[info]   at org.scalatest.concurrent.TimeLimits.failAfter(TimeLimits.scala:231)
[info]   at org.scalatest.concurrent.TimeLimits.failAfter$(TimeLimits.scala:230)
...
[info] *** 24 TESTS FAILED ***
[error] Failed tests:
[error] 	org.apache.spark.sql.connect.pipelines.PythonPipelineSuite
[error] (connect / Test / testOnly) sbt.TestsFailedException: Tests unsuccessful
```

### Does this PR introduce _any_ user-facing change?
No

### How was this patch tested?
- Pass Github Actions
- Manually verify that the relevant tests will no longer fail when PyConnect dependencies are missing.

### Was this patch authored or co-authored using generative AI tooling?
No

Closes #53088 from LuciferYang/SPARK-54375.

Authored-by: yangjie01 <yangjie01@baidu.com>
Signed-off-by: yangjie01 <yangjie01@baidu.com>

Author: LuciferYang

sql/connect/server/src/test/scala/org/apache/spark/sql/connect/pipelines/PythonPipelineSuite.scala

@@ -530,6 +530,7 @@ class PythonPipelineSuite
       "eager analysis or execution will fail")(
     Seq("""spark.sql("SELECT * FROM src")""", """spark.read.table("src").collect()""")) {
     command =>
+      assume(PythonTestDepsChecker.isConnectDepsAvailable)
       val ex = intercept[RuntimeException] {
         buildGraph(s"""
         |@dp.materialized_view
@@ -1047,6 +1048,7 @@ class PythonPipelineSuite
 
   gridTest("Unsupported SQL command outside query function should result in a failure")(
     unsupportedSqlCommandList) { unsupportedSqlCommand =>
+    assume(PythonTestDepsChecker.isConnectDepsAvailable)
     val ex = intercept[RuntimeException] {
       buildGraph(s"""
         |spark.sql("$unsupportedSqlCommand")
@@ -1061,6 +1063,7 @@ class PythonPipelineSuite
 
   gridTest("Unsupported SQL command inside query function should result in a failure")(
     unsupportedSqlCommandList) { unsupportedSqlCommand =>
+    assume(PythonTestDepsChecker.isConnectDepsAvailable)
     val ex = intercept[RuntimeException] {
       buildGraph(s"""
         |@dp.materialized_view()