fix(chain,core): prevent serde from implicitly leaking std into no_std environments

[EliteCoder18]

Description

Fix silent serde/std leak in bdk_core and bdk_chain both crates declared their serde optional dependency without default features = false, causing serde's std feature to activate unconditionally even in no_std builds.

• bdk_core: added default-features = false + serde?/std in std feature.
• Dropped rc the only two serde-derived types (BlockId, ConfirmationBlockTime) are pure scalars needing neither serde/rc nor serde/alloc.
• bdk_chain: added default-features = false and explicitly enabledalloc and rc directly on the serde dep + serde?/std in std. Both are required because tx_graph::ChangeSet holds BTreeSet<Arc<Transaction>> and serde gates Arc<T>: Serialize behind all(feature = "rc", any(feature = "std", feature = "alloc")).

This fix was identified while working on (bitcoindevkit/bdk_wallet#494).

Notes to the reviewers

The alloc and rc features are placed on the serde dependency directly rather than behind a separate feature flag because bdk_chain unconditionally requires alloc anyway.

Changelog notice

### Fixed

- fix(core): serde dep was missing `default-features = false`, leaking `serde/std` into no_std builds and remove unnecessary `rc` feature .
- fix(chain): same leak fixed and `serde/alloc` and `serde/rc` now explicitly enabled on the dep .

Checklists

All Submissions:

• I followed the contribution guidelines

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.84%. Comparing base (081e038) to head (0790f04).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2227   +/-   ##
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  Coverage   78.84%   78.84%           
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  Files          31       31           
  Lines        6060     6060           
  Branches      288      288           
=======================================
  Hits         4778     4778           
  Misses       1203     1203           
  Partials       79       79           

Flag	Coverage Δ
rust	78.84% <ø> (ø)

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[notmandatory]

ACK cc4cef4

Good catch finding that rc isn't needed for bdk_core.

[oleonardolima]

utACK cc4cef4

I merged the update of rustc, so you'll need to rebase it run with the latest CI.

[oleonardolima]

utACK 29d1f5b

[evanlinjin]

ACK 29d1f5b

[notmandatory]

@EliteCoder18 can you please rebase this one? that should fix the required CI checks. Then it looks like it's ready to merge. Thanks!

[EliteCoder18]

@notmandatory Rebased onto the latest master.