fix(cpp): preserve Gorilla sentinel values in batch reads

[ColinLeeo]

Description

Fixes premature termination in C++ Gorilla batch decoding when an encoded value matches the codec end marker.

Gorilla uses canonical NaN as the FLOAT/DOUBLE end marker and reserved minimum values for INT32/INT64. The regular batch API cannot distinguish an embedded marker from the physical end of the stream. Readers already know the exact value count from page metadata or the corresponding time batch, so this change adds exact-count read and skip APIs that preserve embedded marker values while retaining the existing batch fast path.

Changes

• Add shared exact-count read and skip helpers to Decoder.
• Use exact-count decoding in aligned and non-aligned chunk readers.
• Preserve embedded Gorilla marker values and all following values.
• Propagate decoding errors from filtered skip paths.
• Add codec-level and reader-level regression coverage.

Tests

• Full C++ test suite: 733 tests passed.
• Gorilla codec regression for embedded NaN read and skip.
• Aligned and non-aligned DOUBLE/GORILLA reader regression.

[Copilot]

Pull request overview

This PR fixes a correctness issue in the C++ Gorilla decoding path where in-band sentinel values (canonical NaN for FLOAT/DOUBLE, min values for INT32/INT64) could prematurely terminate batch decoding. It introduces exact-count read/skip helpers in the base Decoder so callers that already know the expected value count (via page metadata/bitmaps/time batch counts) can decode deterministically without treating embedded sentinel bit-patterns as end-of-stream.

Changes:

• Add read_exact_* / skip_exact_* helpers to storage::Decoder to enforce exact-count semantics even when batch decoders stop on in-band terminators.
• Switch aligned and non-aligned chunk reader fast paths from capacity-based batch reads/skips to exact-count reads/skips for fixed-width value columns.
• Add regression tests covering embedded Gorilla NaN values at both the codec level and end-to-end reader level.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
cpp/src/encoding/decoder.h	Adds exact-count read/skip helpers that combine batch decoding with scalar fallback to preserve embedded terminator values.
cpp/src/reader/chunk_reader.cc	Uses exact-count skip/read for fixed-width value decoding to prevent Gorilla sentinels from truncating batches.
cpp/src/reader/aligned_chunk_reader.cc	Uses exact-count skip/read for aligned value decoding and predecode paths where the bitmap provides the exact non-null value count.
cpp/test/encoding/gorilla_codec_test.cc	Adds regression coverage for exact-count read/skip preserving embedded NaN values.
cpp/test/reader/tsfile_reader_test.cc	Adds end-to-end reader regression ensuring aligned/non-aligned Gorilla DOUBLE preserves embedded NaN and subsequent values.

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