zephyr: fix pthread stack pool overflow on reconnect (#1064)

[steils]

On Zephyr, _z_task_init() assigns preallocated pthread stacks from thread_stack_area[]. During repeated link resets/reconnects, zenoh-pico recreates the read/lease threads.
Previous implementation used a constantly increasing thread_index++, which eventually indexed past thread_stack_area[], corrupting memory and causing crashes.

Replace thread_index++ with a stack-slot pool. When attr == NULL, pick a free slot in thread_stack_area[], set it with pthread_attr_setstack(), and start the thread.
Slot release is now done with a thread-specific data key destructor.

Add CONFIG_ZENOH_PICO_ZEPHYR_THREADS_NUM (default 4) to configure the stack pool size.

Fixes: #1064

Reproducing

1. Create a new Zephyr project according to Zenoh-Pico Readme, with the following main.c:

#include <stdio.h>
#include <string.h>
#include <zenoh-pico.h>
#include <unistd.h>

#define MODE "client"
#define LOCATOR "tcp/192.168.11.1:7447"

#define KEY_SUB "demo/example/h753/sub"

static void sub_handler(z_loaned_sample_t *s, void *arg) {
    (void)arg;
    z_view_string_t k;
    z_keyexpr_as_view_string(z_sample_keyexpr(s), &k);
    z_owned_string_t v;
    z_bytes_to_string(z_sample_payload(s), &v);
    printf("[sub] %.*s = %.*s\n",
           (int)z_string_len(z_loan(k)), z_string_data(z_loan(k)),
           (int)z_string_len(z_loan(v)), z_string_data(z_loan(v)));
    z_drop(z_move(v));
}

int main(void) {
    printf("zenoh-pico reconnection reproduction start\n");

    z_owned_config_t cfg;
    z_config_default(&cfg);
    zp_config_insert(z_loan_mut(cfg), Z_CONFIG_MODE_KEY, MODE);
    if (strlen(LOCATOR) > 0) {
        zp_config_insert(z_loan_mut(cfg), Z_CONFIG_CONNECT_KEY, LOCATOR);
    }

    z_owned_session_t sess;
    if (z_open(&sess, z_move(cfg), NULL) < 0) {
        printf("Unable to open session\n");
        return -1;
    }
    printf("Session opened\n");

    zp_start_read_task(z_loan_mut(sess), NULL);
    zp_start_lease_task(z_loan_mut(sess), NULL);

    z_view_keyexpr_t ke_sub;
    z_view_keyexpr_from_str_unchecked(&ke_sub, KEY_SUB);
    z_owned_closure_sample_t sub_cb;
    z_closure(&sub_cb, sub_handler, NULL, NULL);
    z_owned_subscriber_t sub;
    if (z_declare_subscriber(z_loan(sess), &sub, z_loan(ke_sub), z_move(sub_cb), NULL) < 0) {
        printf("Unable to declare subscriber\n");
        return -2;
    }
    printf("Subscriber declared on %s\n", KEY_SUB);

    for (int tick = 0;; ++tick) {
        printf("alive tick=%d\n", tick);
        sleep(1);
    }
    return 0;
}

2. Connect the board and run:

pio run
pio run -t upload

3. Verify messages arrive:
   [sub] demo/example/h753/sub = ...

4. Reproduce reconnection:

   • unplug Ethernet cable for ~3-5 seconds
   • plug it back in
   • repeat 4-5 times

Expected result before the fix (fail)

After several reconnects, the firmware will crash due to corrupted stack like in issue #1064.

Expected result (pass)

• No crashes
• The board keeps printing alive tick=...
• After each reconnect, the subscriber resumes receiving [sub] ... messages.
• With -DZENOH_LOG_DEBUG, you may also see zenoh-pico debug logs; there should be no "slot OOM" errors.

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