BigQuery: refactor header handling

[ilidemi]

Have one header provider that always returns a fresh token instead of keeping them per-object and mutating in place
Unblocks bringing the buffered channel back

Closes #3828, a follow up to #3782

[codecov]

❌ 2 Tests Failed:

Tests completed	Failed	Passed	Skipped
1597	2	1595	144

View the top 2 failed test(s) by shortest run time

github.com/PeerDB-io/peerdb/flow/e2e::TestPeerFlowE2ETestSuitePG

Stack Traces | 0.01s run time

=== RUN   TestPeerFlowE2ETestSuitePG
=== PAUSE TestPeerFlowE2ETestSuitePG
=== CONT  TestPeerFlowE2ETestSuitePG
--- FAIL: TestPeerFlowE2ETestSuitePG (0.01s)
2026/01/08 10:09:53 INFO fetched schema x-peerdb-additional-metadata={Operation:FLOW_OPERATION_UNKNOWN} table=e2e_test_mychcl_s6kmbdmn.test_extra_ch_cols

github.com/PeerDB-io/peerdb/flow/e2e::TestPeerFlowE2ETestSuitePG/Test_Dynamic_Mirror_Config_Via_Signals

Stack Traces | 79.2s run time

=== RUN   TestPeerFlowE2ETestSuitePG/Test_Dynamic_Mirror_Config_Via_Signals
=== PAUSE TestPeerFlowE2ETestSuitePG/Test_Dynamic_Mirror_Config_Via_Signals
=== CONT  TestPeerFlowE2ETestSuitePG/Test_Dynamic_Mirror_Config_Via_Signals
    postgres_test.go:922: Inserted 18 rows into the source table
2026/01/08 10:08:28 INFO Executing and processing query x-peerdb-additional-metadata={Operation:FLOW_OPERATION_UNKNOWN} partitionId=testpart query="SELECT id,name,created_at FROM e2e_test_bq_xqaapbcr_20260108100732.\"test_inheritance_dynconf\" ORDER BY id"
2026/01/08 10:08:28 INFO Executing and processing query stream x-peerdb-additional-metadata={Operation:FLOW_OPERATION_UNKNOWN} partitionId=testpart query="SELECT id,name,created_at FROM e2e_test_bq_xqaapbcr_20260108100732.\"test_inheritance_dynconf\" ORDER BY id"
2026/01/08 10:08:28 INFO [pg_query_executor] declared cursor x-peerdb-additional-metadata={Operation:FLOW_OPERATION_UNKNOWN} partitionId=testpart cursorQuery="DECLARE peerdb_cursor_8692905791343598008 CURSOR FOR SELECT id,name,created_at FROM e2e_test_bq_xqaapbcr_20260108100732.\"test_inheritance_dynconf\" ORDER BY id" args=[]
2026/01/08 10:08:28 INFO [pg_query_executor] fetching rows start x-peerdb-additional-metadata={Operation:FLOW_OPERATION_UNKNOWN} partitionId=testpart query="SELECT id,name,created_at FROM e2e_test_bq_xqaapbcr_20260108100732.\"test_inheritance_dynconf\" ORDER BY id" channelLen=0
2026/01/08 10:08:28 INFO [pg_query_executor] fetching from cursor x-peerdb-additional-metadata={Operation:FLOW_OPERATION_UNKNOWN} partitionId=testpart cursor=peerdb_cursor_8692905791343598008
2026/01/08 10:08:28 INFO processed row stream x-peerdb-additional-metadata={Operation:FLOW_OPERATION_UNKNOWN} partitionId=testpart cursor=peerdb_cursor_8692905791343598008 records=8 bytes=168 channelLen=7
2026/01/08 10:08:28 INFO [pg_query_executor] fetched rows x-peerdb-additional-metadata={Operation:FLOW_OPERATION_UNKNOWN} partitionId=testpart query="SELECT id,name,created_at FROM e2e_test_bq_xqaapbcr_20260108100732.\"test_inheritance_dynconf\" ORDER BY id" rows=8 bytes=168 channelLen=0
2026/01/08 10:08:28 INFO [pg_query_executor] fetching from cursor x-peerdb-additional-metadata={Operation:FLOW_OPERATION_UNKNOWN} partitionId=testpart cursor=peerdb_cursor_8692905791343598008
2026/01/08 10:08:28 INFO processed row stream x-peerdb-additional-metadata={Operation:FLOW_OPERATION_UNKNOWN} partitionId=testpart cursor=peerdb_cursor_8692905791343598008 records=0 bytes=0 channelLen=0
2026/01/08 10:08:28 INFO [pg_query_executor] fetched rows x-peerdb-additional-metadata={Operation:FLOW_OPERATION_UNKNOWN} partitionId=testpart query="SELECT id,name,created_at FROM e2e_test_bq_xqaapbcr_20260108100732.\"test_inheritance_dynconf\" ORDER BY id" rows=0 bytes=0 channelLen=0
2026/01/08 10:08:28 INFO [pg_query_executor] committing transaction x-peerdb-additional-metadata={Operation:FLOW_OPERATION_UNKNOWN} partitionId=testpart
2026/01/08 10:08:28 INFO [pg_query_executor] committed transaction for query x-peerdb-additional-metadata={Operation:FLOW_OPERATION_UNKNOWN} partitionId=testpart query="SELECT id,name,created_at FROM e2e_test_bq_xqaapbcr_20260108100732.\"test_inheritance_dynconf\" ORDER BY id" rows=8 bytes=168 channelLen=0
    postgres_test.go:922: Inserted 18 rows into the source table
    postgres_test.go:931: WaitFor normalize 18 records - first table 2026-01-08 10:08:32.205161181 +0000 UTC m=+218.099067980
    postgres_test.go:931: UNEXPECTED TIMEOUT normalize 18 records - first table 2026-01-08 10:09:32.299196529 +0000 UTC m=+278.193103325
    postgres.go:92: begin tearing down postgres schema pg_z13vfppu
--- FAIL: TestPeerFlowE2ETestSuitePG/Test_Dynamic_Mirror_Config_Via_Signals (79.23s)

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[jkaflik]

cool change @ilidemi